Laminar to Turbulent Transition

In this work the laminar-to-turbulent transition in microchannels of circular cross-section is studied experimentally. In order to single out the effects of relative roughness, compressibility and channel length-to-diameter ratio on the Reynolds number at which transition occurs, experimental runs have been carried out on circular microchannels in fused silica-smooth for all purposes-and in stainless steel (which possess a high surface roughness), with a diameter between 125 and 180 lm and a length of 5-50 cm through which nitrogen flows. For each tube the friction factor has been computed. The values of the critical Reynolds number have been determined plotting the Poiseuille number (i.e., the product of the friction factor, f, times the Reynols number, Re) as a function of the average Mach number between inlet and outlet. The transitional regime was found to start no earlier than at values of the Reynolds number around 1,800-2,000. It has been observed that surface roughness has no effect on the hydraulic resistance in the laminar region for a relative roughness lower than 4.4%, and that friction factor obeys the Poiseuille law, if it is correctly computed taking compressibility into account. It is found that recent correlations for the prediction of the critical Reynolds number in microchannels that link the relative roughness of the microtubes to the critical Reynolds number do not agree with the present results.

An algebraic intermittency model for boundary layer flow transition from laminar to turbulent state, is extended using an experimental data base on boundary layer flows with various transition types and results by large eddy simulation of transition in a separated boundary layer. The originating algebraic transition model functions well for bypass transition in an attached boundary layer under a moderately high or elevated free-stream turbulence level, and for transition by Kelvin-Helmholtz instability in a separated boundary layer under a low free-stream turbulence level. It also functions well for transition in a separated layer, caused by a very strong adverse pressure gradient under a moderately high or elevated free-stream turbulence level. It is not accurate for transition in a separated layer under a moderately strong adverse pressure gradient, in the presence of a moderately high or elevated free-stream turbulence level. The extension repairs this deficiency. Therefore, a sensor function for detection of the front part of a separated boundary layer activates two terms that express the effect of free-stream turbulence on the breakdown of a separated layer, without changing the functioning of the model in other flow regions. The sensor and the breakdown terms use only local variables.

The predictive qualities of a recently developed algebraic intermittency model for laminar-to-turbulent transition are analysed for the flow through a linear cascade of low-pressure turbine blades with an endwall. Both steady RANS (Reynolds-averaged Navier-Stokes) and time-accurate RANS (URANS) simulations are performed. The results are compared with reference LES (Large Eddy Simulation) by Cui et al. (2017, Numerical investigation of secondary flows in a high-lift low pressure turbine, Int. J. of Heat and Fluid Flow, vol. 63) and results by the local correlation-based intermittency transport model (LCTM) by Menter et al. (2015, A oneequation local correlation-based transition model. Flow Turbul. Combust., vol. 95) for laminar and turbulent endwall boundary layers at the cascade entrance. Good agreement is obtained with the reference LES and with results by the LCTM for the evolution through the cascade of the mass-averaged total pressure loss coefficient and for profiles of pitchwise-averaged total pressure loss coefficient at the cascade exit.

The predictive qualities of a recently developed algebraic intermittency model for laminar-to-turbulent transition are analysed for the flow through a linear cascade of low-pressure turbine blades with an endwall. Both steady RANS (Reynolds-averaged Navier-Stokes) and time-accurate RANS (URANS) simulations are performed. The results are compared with reference LES (Large Eddy Simulation) by Cui et al. (2017, Numerical investigation of secondary flows in a high-lift low pressure turbine, Int. J. of Heat and Fluid Flow, vol. 63) and results by the local correlation-based intermittency transport model (LCTM) by Menter et al. (2015, A oneequation local correlation-based transition model. Flow Turbul. Combust., vol. 95) for laminar and turbulent endwall boundary layers at the cascade entrance. Good agreement is obtained with the reference LES and with results by the LCTM for the evolution through the cascade of the mass-averaged total pressure loss coefficient and for profiles of pitchwise-averaged total pressure loss coefficient at the cascade exit.

The prediction of hypersonic boundary-layer transition location from a laminar to a turbulent state is vital to the development of hypersonic vehicles because of the first-order impact on aerodynamic heating, drag force, engine performance, and vehicle operation.

The empirical correlation for prediction of the onset of boundary layer transition, presented previously by authors, Taghavi et al.(2009), was extended by taking into account the effects of both the streamwise pressure gradients and the freestream turbulence intensity. The correlation was derived based on experimental data of transitional boundary layers subjected to different freestream turbulence intensities and streamwise pressure gradients, obtained in this study as well as from other researchers. The experiments were carried out in two different free stream turbulence levels and four different pressure gradients. A single hot-wire probe was used for measurements of instantaneous flow velocities within the boundary layer. Experiments were conducted in an open circuit wind tunnel of suction type. For each pressure gradient in a distinct free stream turbulence level, the mean longitudinal velocity and root mean square, RMS, of the velocity fluctuations were obtained at several str...

An original technology of stability analysis of an airfoil boundary layer is presented in this paper. The technology includes an extraction of the boundary layer from the results of laminar _ow computations, the computation of neutral curves and increments of Tollmien–Schlichting waves, and the computation of the location of laminar-turbulent transition zone by the eN-method for a given set of freestream turbulence levels. To be speci_c,we consider the _ow around the NACA23012 airfoil with zero angle of attack at moderate Reynolds numbers.

Integral calculations of two-dimensional, incompressible, thermal, transitional boundary layers have been performed. To precede these approximate calculations, mathematical model was developed in order to enable prediction of the main boundary layer integral parameters. The model was proposed to calculate the characteristics of the boundary layers under the effect of local heat transfer and moderate free-stream turbulence levels by enhancing established integral techniques in conjunction with intermittency weighted model of the transitional boundary layer. Empirical relationships for the prediction of the start and end of transition, as well as the development of the boundary layer during the transition region were based on results of experimental investigations. Since the heat transfer coefficient between external flow and surface is extremely influenced by the level of turbulence in the flow, it is also found to be very sensitive to the solid surface temperature and thereby an ade...

The laminar-turbulent transition has significant influence on determining the aerodynamic characteristics of immersed bodies such as evolution of losses, the appearance of separation and stall. Also, the transition behavior has a dominant effect on the distributions of wall shear stress and surface heat transfer. To predict and manage the turbulence in different flow cases is beneficial for optimum advantage, namely, to reduce it when it is harmful (e.g. to decrease the skin friction or heat transfer) and to increase it when it is desirable (to avoid flow separation). The prediction of transition at high free-stream turbulence is of particular importance in turbomachinery where the boundary layer state defines the blade heat transfer and the flow separation margins. This project report presents a study on measurements and prediction of laminarturbulent transition at high free-stream turbulence in boundary layers of the airfoillike geometries with presence of the external pressure gradient changeover. The experiments are performed for a number of flow cases with different flow Reynolds number, turbulence intensity and pressure gradient distributions. The results were then compared to numerical calculations for same geometries and flow conditions. The experiments and computations are performed for the flow parameters which are typical for turbomachinery applications and the major idea of current study is the validation of the turbulence model which can be used for such engineering applications.

Heat transfer study on gas turbine blades is very important due to the resultant increase in cycle thermal efficiency. This study is focused on the heat transfer effects on a reference nozzle guide vane and test rig component preparation in heat and power technology division at KTH. In order to prepare the current test rig for heat transfer experiments, some feature should be changed in the current layout to give a nearly instant temperature rise for heat transfer measurement. The heater mesh component is the main component to be added to the current test rig. Some preliminary design parameters were set and the necessary power for the heater mesh to achieve required step temperature rise was calculated. For the next step, it is needed to estimate the heat transfer coefficient and the other parameters for study on the reference blade using numerical methods. Boundary layer analysis is very important in heat transfer modeling so to model the reference blade heat transfer and boundary ...

Abstract: We report here the results of a study on measurements and prediction of laminar-turbulent transition at high free-stream turbulence in boundary layers of the airfoil-like geometries with presence of the external pressure gradient changeover. The experiments are performed for a number of flow cases with different flow Reynolds number, turbulence intensity and pressure gradient distributions. The results were then compared to numerical calculations for same geometries and flow conditions. The experiments and computations are performed for the flow parameters which are typical for turbomachinery applications and the major idea of the current study is the validation of the turbulence model which can be used for such engineering applications. 1. INTRODUCTION The laminar-turbulent transition has significant influence on determining the aerodynamic characteristics of immersed bodies such as evolution of losses, the appearance of separation and stall. Also, the transition behaviou...

The separation of unsteadiness (or intermittency) and turbulence is the key to improving understanding of the statistical behavior of the transitional boundary layer flow. In this study an attempt is made to review the different methods used for the detection of turbulent and non-turbulent interface in transitional flows. In this regard, methods for generating the intermittency function, conditional averaging and sampling are discussed. The salient features of each technique is also explained.

The separation of unsteadiness (or intermittency) and turbulence is the key to improving understanding of the statistical behavior of the transitional boundary layer flow. In this study an attempt is made to review the different methods used for the detection of turbulent and non-turbulent interface in transitional flows. In this regard, methods for generating the intermittency function, conditional averaging and sampling are discussed. The salient features of each technique is also explained.

We report here the results of a study on measurements and prediction of laminar-turbulent transition at high free-stream turbulence in boundary layers of the airfoil-like geometries with presence of the external pressure gradient changeover. The experiments are performed for a number of flow cases with different flow Reynolds number, turbulence intensity and pressure gradient distributions. The results were then compared to numerical calculations for same geometries and flow conditions. The experiments and computations are performed for the flow parameters which are typical for turbomachinery applications and the major idea of the current study is the validation of the turbulence model which can be used for such engineering applications.

A detailed investigation to document momentum and thermal development of boundary layers undergoing natural transition on a heated flat plate was performed. Experimental results of both overall and conditionally sampled characteristics of laminar, transitional and low Reynolds number turbulent[boundary layers are presented. Measurements were done in a low-speed, closed-loop wind tunne1_with a freestream velocity of I00 ft/s and zero pressure gradient over a range of freestream turb_lence intensities from 0.4 to 6 percent. The distributions of skin friction, heat transfer rate and Reynolds shear stress were a11 consistent with previously published data. Reynolds analogy factors for momentum thickness Reynolds number, Re8 < 2300 were found to be well predicted by laminar and turbulent correlations which accounted for an unheated starting length and uniform heat flux. A small dependence of turbulent results on the freestream turbulence intensity was observed. 19. Security Classif. (of the report) 20, Security Classif. (of this page) 21. No. of pages Unclassified Unclassified 16 NASA FORM 1626 OCT 88

• A three-wire probe was specifically designed and fabricated to measure the flow structure and the thermal field in transitional and low Reynolds number turbulent flow. In addition to the skin friction coefficient and Stanton number on the surface, detailed measurements were made of the mean and fluctuation quantities of the flow and temperature fields in the boundary layer. The evolution of the uv profiles indicates that turbulent shear is generated near Y+= 70-100 and imposes itself on the wall within the boundary layer, ut, vt, e H, eM, and Pr t were measured near the end of transition and in the turbulent flow region. Pr t is shown to be greater than the typical value of 0.9 in the near-wall region. The Reynolds analogy factor (2 St/Cf) in the early turbulent flow region is approximately 0.9, which is lower than the typical value of 1.2 for turbulent flow. The data presented in this paper serve as the baseline for a series of consecutive studies related to investigating various parameters that affect laminar-turbulent transition.

The separation of unsteadiness (or intermittency) and turbulence is the key to improving understanding of the statistical behavior of the transitional boundary layer flow. In this study an attempt is made to review the different methods used for the detection of turbulent and non-turbulent interface in transitional flows. In this regard, methods for generating the intermittency function, conditional averaging and sampling are discussed. The salient features of each technique is also explained.

Transition modeling as applied to CFD methods has followed certain line of evolution starting from simple linear stability methods to almost or fully predictive methods such as LES and DNS. One pragmatic approach among these methods, such as the local correlation-based transition modeling approach, is gaining more popularity due to its straightforward incorporation into RANS solvers. Such models are based on blending the laminar and turbulent regions of the flow field by introducing intermittency equations into the turbulence equations. Menter et al. pioneered this approach by their two-equation γ-Reθ intermittency equation model that was incorporated into the k-ω SST turbulence model that results in a total of four equations. Later, a range of various three-equation models was developed for super-/hypersonic flow applications. However, striking the idea that the Reθ-equation was rather redundant, Menter produced a novel one-equation intermittency transport γ-equation model. In this report, yet another recently introduced transition model called as the Bas-Cakmakcioglu (B-C) algebraic model is elaborated. In this model, an algebraic γ-function, rather than the intermittency transport γ-equation, is incorporated into the one-equation Spalart-Allmaras turbulence model. Using the present B-C model, a number of two-dimensional test cases and three-dimensional test cases were simulated with quite successful results.

Space air diffusion systems are an integral part of many HVAC systems 1-2. The design of the system includes finding the best location for introduction of air into the room. This paper expounds on the designing, building, and testing of a space air diffusion laboratory setup for undergraduate engineering students. The laboratory will enable students to conduct hands-on experiments that involve visualization and measurements of laminar, transitional, and turbulent ceiling wall air-jets and the resulting room air motion. Students designed the space air diffusion experimental test setup for use in the Fluid Mechanics course's laboratory and for the ASHRAE Senior Undergraduate Project Grant program, which funded the project. Three groups of students in the Manufacturing Processes course designed the experimental setupone group designed the mechanism to position the Pitot-tube and hot-wire anemometry measurement devices in the flow field, another group designed the layout of the channel the air would pass through to become steady flow, and the remaining group designed the section which would streamline the air supplied by the fan. The overall objective was to engage the students in a design project. This paper will provide details of the evaluations and outcomes of the project. For this project, the students designed the apparatus using SolidWorks® CAD software, and ANSYS Fluent software was used for CFD simulations of the flow field. The components used in the design were: a blower, a settling chamber, a perforated plate, honeycomb, mesh screens, and a 3D printed contraction in the shape of a fifth order polynomial to minimize the turbulence level of the flow entering the 2,440 mm long plane channel made with a cross section of 10 mm high and 232 mm wide. The purpose of the channel was to get a fully developed flow entering the model room for calibration of the hot-wire anemometer. The dimensions of the model room designed by the students are 232 mm in height, 232 mm in width, and 464 mm in length. The ceiling wall jet was discharged parallel and adjacent to the straight horizontal ceiling of the model room. The jet developed along the ceiling surface of the room and entrained air from the room as its velocity deceased when it moved into the room. The maximum velocity remained close to the ceiling surface with the distance that the ceiling jet adhered to the surface depending on the relative influence of inertia and gravity.

In gas turbine engines, laminar-turbulent transition occurs. However, generally, the turbulence models to describe such transition results in too early and too short transition. Combining a turbulence model with a description of intermittency, i.e. the fraction of time the flow is turbulent during the transition phase, can improve it. By letting grow the intermittency from zero to unity, start and evolution of transition can be imposed. In this paper, a method where a dynamic equation of intermittency combining with a two-equation k-c0 turbulence model is described. This intermittency factor is a premultiplicator of the turbulent viscosity computed by the turbulence model. Following a suggestion by Menter et alJ 11, the start of transition is computed based on local variables.

Prediction of laminar-turbulent transition at high level of free-stream turbulence in boundary layers of airfoil geometries with external pressure gradient changeover is in focus. The aim is a validation of a transition model for transition prediction in turbomachinery applications. Numerical simulations have been performed by using a transition model by Langtry and Menter for a number of different cases of pressure gradient, at Reynolds number-range, based on the airfoil chord, 50 000 ≤ Re ≤ 500 000 and free-stream turbulence intensities 2 % and 4 %. The validation of the computational results against the experimental data showed good performance of used turbulence model for all test cases.

We report here the results of a study on measurements and prediction of laminar-turbulent transition at high free-stream turbulence in boundary layers of the airfoil-like geometries with presence of the external pressure gradient changeover. The experiments are performed for a number of flow cases with different flow Reynolds number, turbulence intensity and pressure gradient distributions. The results were then compared to numerical calculations for same geometries and flow conditions. The experiments and computations are performed for the flow parameters which are typical for turbomachinery applications and the major idea of the current study is the validation of the turbulence model which can be used for such engineering applications.

Present paper is partly a declaration of state of a currently ongoing PhD work about turbulent flow in a thick walled pipe in order to analyze conjugate heat transfer. An ongoing effort on CFD investigation of this problem using cylindrical coordinates and dimensionless governing equations is identified alongside a literature review. The mentioned PhD work will be conducted using an in-house developed code. However it needs preliminary evaluation by means of commercial codes available in the field. Accordingly ANSYS CFD was utilized in order to evaluate mesh structure needs and asses the turbulence models and solution options in terms of computational power versus difference signification. Present work contains a literature survey, an arrangement of governing equations of the PhD work, CFD essentials of the preliminary analysis and findings about the mesh structure and solution options. Mesh element number was changed between 5,000 and 320,000. k-ϵ, k-ω, Spalart-Allmaras and Viscous-Laminar models were compared. Reynolds number was changed between 1,000 and 50,000. As it may be expected due to the literature, k-ϵ yields more favorable results near the pipe axis and k-ω yields more convenient results near the wall. However k-ϵ is found sufficient to give turbulent structures for a conjugate heat transfer problem in a thick walled plain pipe.

In this paper, the effect of one of important parameters and affecting the transition process in the boundary layers, namely, the pressure gradient along to the flow on the transition onset position and length of the transition zone/region, have been numerically investigated. Perot turbulence potential model has been used to capture turbulence and flow transition characteristics. This model has appropriate structural relationships for analyzing non-equilibrium flows, which is a suitable capability for simulating the transitional flows. In this research, the governing equations of this new turbulent model have been applied to the fluent software in the form of programming of UDF and UDS. Numerical analyzes for five types of different flows have been carried out under a wide condition of pressure gradient and turbulence intensity of free flow and compared with experimental data. The results show that increasing the turbulence intensity of the inlet free flow as well as the reverse pressure gradient leads to begin transition onset position sooner and the length of the transition zone is also smaller and vice versa. Another important result is that, in flows with turbulence intensities of more than 5%, the effect of the pressure gradient on the transition onset position in the boundary layers is negligible.

A new transport equation for the intermittency factor was proposed to predict separated and transitional boundary layers under low-pressure turbine airfoil conditions. The intermittent behavior of the transitional flows is taken into account and incorporated into computations by modifying the eddy viscosity, µ t , with the intermittency factor, y. Turbulent quantities are predicted by using Menter's two-equation turbulence model (SST). The intermittency factor is obtained from a transport equation model, which not only can reproduce the experimentally observed streamwise variation of the intermittency in the transition zone, but also can provide a realistic cross-stream variation of the intermittency profile. In this paper, the intermittency model is used to predict a recent separated and transitional boundary layer experiment under low pressure turbine airfoil conditions. The experiment provides detailed measurements of velocity, turbulent kinetic energy and intermittency profiles for a number of Reynolds numbers and freestream turbulent intensity conditions and is suitable for validation purposes. Detailed comparisons of computational results with experimental data are presented and good agreements between the experiments and predictions are obtained.

The nature of flow development in a parallel plate channel has been investigated by making use of a newly developed model of intermittency. That model, taken together with the RANS equations of momentum conservation, the continuity equation, and the SST turbulence model, was employed to provide a complete chronology of the development processes and the derived practical results. A major focus of the work is the effect of inlet conditions on the downstream behavior of the developing flow. It was observed that the flow development process depends critically on the specifics of the inlet conditions characterized here by the shape of the velocity profile and the magnitude of the turbulence intensity. Two velocity profile shapes (flat and parabolic), are regarded as limiting cases. Similarly, two turbulence intensities, Tu = 1% and 5%, are employed. From the standpoint of practice, the relationship between the friction factor and the Reynolds number is most significant. It was found that this relationship reflects that of standard practice for only one of the investigated cases (flat velocity profile, Tu = 5%). For the other cases (flat profile, Tu = 1% and parabolic profile, Tu = 1% and 5%), the breakdown of laminar flow is delayed and the onset of full turbulence occurs rather abruptly at Re $10,000. Three unique fully developed flow regimes are existent, depending on the inlet conditions and on the value of the Reynolds number. In addition to the standard laminar and fully turbulent regimes, another regime, fully developed intermittent, can occur. Specifically, in the latter regime, laminar and turbulent flows occur intermittently.

In this research, an open-circuit wind tunnel with three diameters variation (6 mm , 8.5 mm and 18 mm) of the honeycomb has been made. The objective of the research is to determine the value of turbulence intensity within the tunnel. Wind speed data at a specific time in the tunnel were taken by using anemometer and procceed into the turbulence intensity data. The results show that the turbulence intensity values of the honeycomb diameter of 18 mm and 8.5 mm are 0.17092 and 0.1781, respectively. While the turbulence intensity value of haneycomb diameter of 6 mm is 0.1227. The results of simulations using CFD program show that similarities fluctuations in flow wind velocity with existing experimental results. It can be seen from the color of the indicator fluktuation flow wind velocity on this computational simulation. It is concluded that the diameter of the honeycomb influence the quality of the air flow within the wind tunnel. The turbulence intensity is minimun at a small honeycomb diameter.

Prediction of laminar-turbulent transition at high level of free-stream turbulence in boundary layers of airfoil geometries with external pressure gradient changeover is in focus. The aim is a validation of a transition model for transition prediction in turbomachinery applications. Numerical simulations have been performed by using a transition model by Langtry and Menter for a number of different cases of pressure gradient, at Reynolds number-range, based on the airfoil chord, 50 000 ≤ Re ≤ 500 000 and free-stream turbulence intensities 2 % and 4 %. The validation of the computational results against the experimental data showed good performance of used turbulence model for all test cases.

Experimental measurements of both mean and conditionally sampled characteristics of laminar, transitional and low Reynolds number turbulent boundary layers on a heated flat plate are presented. Measurements were obtained in air over a range of freestream turbulence intensities from 0.3 percent to 6 percent with a freestream velocity of 30.5 m/s and zero pressure gradient. Conditional sampling performed in the transitional boundary layers indicate the existence of a near-wall drop in intermittency, especially pronounced at low intermittencies. Nonturbulent intervals were observed to possess large levels of low-frequency unsteadiness, and turbulent intervals had peak intensities as much as 50 percent higher than were measured at fully turbulent stations. Heat transfer results were consistent with results of previous researches and Reynolds analogy factors were found to be well predicted by laminar and turbulent correlations which accounted for unheated starting length. A small depende...

During an experimental investigation of the transitional boundary layer over a heated flat plate, an unexpected result was encountered for the turbulent heat flux (bar-v&#39;t&#39;). This quantity, representing the correlation between the fluctuating normal velocity and the temperature, was measured to be negative near the wall under certain conditions. The result was unexpected as it implied a counter-gradient heat transfer by the turbulent fluctuations. Possible reasons for this anomalous result were further investigated. The possible causes considered for this negative bar-v&#39;t&#39; were: (1) plausible measurement error and peculiarity of the flow facility, (2) large probe size effect, (3) &#39;streaky structure&#39; in the near wall boundary layer, and (4) contributions from other terms usually assumed negligible in the energy equation including the Reynolds heat flux in the streamwise direction (bar-u&#39;t&#39;). Even though the energy balance has remained inconclusive, non...

In gas turbine engines, laminar-turbulent transition occurs. However, generally, the turbulence models to describe such transition results in too early and too short transition. Combining a turbulence model with a description of intermittency, i.e. the fraction of time the flow is turbulent during the transition phase, can improve it. By letting grow the intermittency from zero to unity, start and evolution of transition can be imposed. In this paper, a method where a dynamic equation of intermittency combining with a two-equation k-c0 turbulence model is described. This intermittency factor is a premultiplicator of the turbulent viscosity computed by the turbulence model. Following a suggestion by Menter et alJ 11, the start of transition is computed based on local variables.

... Keywords: bypass transition, intermittency, SST turbulence model. ... The specification of the MUR test cases is given in Table 2. The incoming turbulence level Tui is measured 55mm upstream of the leading edge. Also the exit Mach number Page 2. Koen LODEFIER et al. ...

An experimental study has been conducted to investigate the effects of oscillation, of varying amplitude (0 to 0.03 m) and frequency (0 to 6 Hz), on the boundary-layer development of a flat plate under the influence of adverse pressure gradients. The Reynolds number was 10 6 based on the flat plate chord length. The study employed a subsonic wind tunnel facility at a freestream velocity of 16 m=s. A cylinder placed at various locations over the flat plate within the boundary layer was used to create the effects of adverse pressure gradients. Surface pressures were measured along the centerline of the flat plate to study the flow quantitatively. Oil flow visualizations were also obtained to examine the flow qualitatively. Boundary-layer surveys were conducted to examine the effect of oscillation on the mean velocity, turbulence intensity, and boundary-layer thickness profiles.

Flow and convective heat transfer characteristics are considered by solving the mass, momentum and energy equations for two dimensional laminar and turbulent boundary layers at zero, favourable and adverse pressure gradients, including the influence of unheated starting length, surface curvature and free stream pressure gradients. The numerical predictions have been compared with approximate methods and experimental results. Results showed that Stanton numbers in laminar flat plates increased in all favourable pressure gradients and but increased only in moderate favourable pressure gradients of concave walls. On the other hand, Stanton numbers decreased both with adverse pressure gradients in laminar flow and with favorable pressures in tubulent flows.

CFD analyses were performed for a cross-flow experiment in prismatic HTGR core fuel blocks to investigate the effect of turbulence models on the prediction of the core flow distribution. A transition model was considered in addition to a fully turbulent model because the flow in the coolant channels of the fuel block is turbulent whereas that in the bypass gap between the blocks is laminar. Additional analyses were carried out to investigate how the variation of the coolant channel flow dependence on the turbulence model selection affected the hot spot fuel temperature. The results showed that the gamma-theta transition model led to a better prediction than the others because it reasonably simulated the flow region where the bypass and cross gaps intersected. The result for the hot spot fuel temperature, however, showed that the temperature change caused by the turbulence model selection is so small that it can be absorbed in a margin of the engineering design. Therefore, the fully turbulent model can be used as an efficient alternative to the transition model in the engineering calculation of an HTGR core flow.

Keywords: high temperature gas-cooled reactor (HTGR), prismatic block core, bypass flow, fuel temperature, laminar and turbulent flows

The laminar-turbulent transition has significant influence on determining the aerodynamic characteristics of immersed bodies such as evolution of losses, the appearance of separation and stall. Also, the transition behavior has a dominant effect on the distributions of wall shear stress and surface heat transfer. To predict and manage the turbulence in different flow cases is beneficial for optimum advantage, namely, to reduce it when it is harmful (e.g. to decrease the skin friction or heat transfer) and to increase it when it is desirable (to avoid flow separation). The prediction of transition at high free-stream turbulence is of particular importance in turbomachinery where the boundary layer state defines the blade heat transfer and the flow separation margins.

We report here the results of a study on measurements and prediction of laminar-turbulent transition at high free-stream turbulence in boundary layers of the airfoil-like geometries with presence of the external pressure gradient changeover. The experiments are performed for a number of flow cases with different flow Reynolds number, turbulence intensity and pressure gradient distributions. The results were then compared to numerical calculations for same geometries and flow conditions. The experiments and computations are performed for the flow parameters which are typical for turbomachinery applications and the major idea of the current study is the validation of the turbulence model which can be used for such engineering applications.

The separation of unsteadiness (or intermittency) and turbulence is the key to improving understanding of the statistical behavior of the transitional boundary layer flow. In this study an attempt is made to review the different methods used for the detection of turbulent and non-turbulent interface in transitional flows. In this regard, methods for generating the intermittency function, conditional averaging and sampling are discussed. The salient features of each technique is also explained.

This paper is focused on measurements and prediction of laminar-turbulent transition at high free-stream turbulence in boundary layers of airfoil geometries with external pressure gradient changeover. Experimental and numerical study is performed for a number of flow cases covering a range of flow Reynolds numbers, turbulence intensities and pressure gradient distributions. The flow parameters in experiments and computations are typical for turbomachinery applications and main motivation of current study is validation of transition models which can be used for transition prediction in such engineering applications. In current work the experimental data are used to validate a transition model by Langtry and Menter which showed good agreement with experiments for all test cases.

The distinguishing features of transition in turbomachinery flows are that they are unsteady on time scales longer than typical eddy turnover times, occur in harsh and highly disturbed environments at awkward Reynolds numbers, and are generally three-dimensional (even in the mean). As a consequence, the flow is a veritable-fluid-dynamical 'zoo', characterized by separation, reattachment, transition, relaminarization , retransition, etc., all often occurring in the same flow. But how important is transition research for the turbomachinery industry? GE compressor tests (made by Halstead) showing transition extending over 60% of the blade chord, and estimates of potential improvement inefficiency by several percentage points; considering how widely turbomachines are used in energy conversion and propulsion systems, significant economic and environmental benefits are possible. It is found out that the 'lack of ability to predict the location of boundary layer transition for components in gas turbine engines is impeding our ability to gain maximum benefit from our design effort. If a complete computational fluid dynamics (CFD) design tool incorporating transition were to be available, it is foreseen airfoil designs with higher blade loading that would reduce part count and improve efficiency. It is estimated that a 1% improvement in the efficiency of a low pressure turbine would result in a saving of $52,000 per year on a typical airliner. Improved transition technology was thus very relevant. So keeping in mind such important issue like transition phenomena in design and operation of gas turbines, it is the aim of this review paper to elucidate the recent research activities in this area.