Implicit-LES Simulation of Variable-Speed Power Turbine Cascade for Low Free-Stream Turbulence Conditions

2018 ◽  
Author(s):  
Ali Ameri
Keyword(s):  
Free Stream ◽  
Reynolds Numbers ◽  
Scale Model ◽  
Variable Speed ◽  
Low Reynolds Numbers ◽  
Free Stream Turbulence ◽  
Pressure Loss Coefficient ◽  
Power Turbine ◽  
Actual Design ◽  
Low Reynolds

It is a challenge to simulate the flow in a Variable Speed Power Turbine (VSPT), or, for that matter, rear stages of low pressure turbines at low Reynolds numbers due to laminar flow separation or laminar/turbulent flow transition on the blades. At low Reynolds numbers, separation induced-transition is more prevalent which can result in efficiency lapse. LES has been used in recent years to simulate these types of flows with a good degree of success. In the present work, very low free stream turbulence flows at exit Reynolds number of 220k were simulated. The geometry was a cascade which was constructed with the midspan section of a VSPT design. Most LES simulations to date, have focused on the midspan region. As the endwall effect was significant in these simulations due to thick incoming boundary layer, full blade span computation was necessitated. Inlet flow angles representative of take-off and cruise conditions, dictated by the rotor speed in an actual design, were analyzed. This was done using a second order finite volume code and a high resolution grid. As is the case with Implicit-LES methods, no sub-grid scale model was used. Blade static pressure data, at various span locations, and downstream probe survey measurements of total pressure loss coefficient were used to verify the results. The comparisons showed good agreement between the simulations and the experimental data.

Measurements in a Turbine Cascade Flow Under Ultra Low Reynolds Number Conditions

2001 ◽  
Vol 124 (1) ◽  
pp. 100-106 ◽  
Author(s):  
Kenneth W. Van Treuren ◽  
Terrence Simon ◽  
Marc von Koller ◽  
Aaron R. Byerley ◽  
James W. Baughn ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Reynolds Numbers ◽  
Vortex Generators ◽  
Separation Region ◽  
Suction Surface ◽  
Low Reynolds Numbers ◽  
Free Stream Turbulence ◽  
Low Reynolds

With the new generation of gas turbine engines, low Reynolds number flows have become increasingly important. Designers must properly account for transition from laminar to turbulent flow and separation of the flow from the suction surface, which is strongly dependent upon transition. Of interest to industry are Reynolds numbers based upon suction surface length and flow exit velocity below 150,000 and as low as 25,000. In this paper, the extreme low end of this Reynolds number range is documented by way of pressure distributions, loss coefficients, and identification of separation zones. Reynolds numbers of 25,000 and 50,000 and with 1 percent and 8-9 percent turbulence intensity of the approach flow (free-stream turbulence intensity, FSTI) were investigated. At 25,000 Reynolds number and low FSTI, the suction surface displayed a strong and steady separation region. Raising the turbulence intensity resulted in a very unsteady separation region of nearly the same size on the suction surface. Vortex generators were added to the suction surface, but they appeared to do very little at this Reynolds number. At the higher Reynolds number of 50,000, the low-FSTI case was strongly separated on the downstream portion of the suction surface. The separation zone was eliminated when the turbulence level was increased to 8-9 percent. Vortex generators were added to the suction surface of the low-FSTI case. In this instance, the vortices were able to provide the mixing needed to re-establish flow attachment. This paper shows that massive separation at very low Reynolds numbers (25,000) is persistent, in spite of elevated FSTI and added vortices. However, at a higher Reynolds number, there is opportunity for flow reattachment either with elevated free-stream turbulence or with added vortices. This may be the first documentation of flow behavior at such low Reynolds numbers. Although it is undesirable to operate under these conditions, it is important to know what to expect and how performance may be improved if such conditions are unavoidable.

Effects of Free Stream Turbulence on Low Reynolds Number Boundary Layers

1984 ◽  
Vol 106 (3) ◽  
pp. 298-306 ◽  
Author(s):  
I. P. Castro
Keyword(s):  
Reynolds Number ◽  
Boundary Layers ◽  
Free Stream ◽  
Reynolds Numbers ◽  
Length Scale ◽  
Mean Flow ◽  
Low Reynolds Numbers ◽  
Free Stream Turbulence ◽  
Reynolds Number Effects ◽  
Low Reynolds

This paper documents some of the effects of free stream turbulence on the mean flow properties of turbulent boundary layers in zero pressure gradients. Attention is concentrated on flows for which the momentum thickness Reynolds number is less than about 2000. Direct Reynolds number effects are therefore significant and it is shown that such effects reduce as the level of free stream turbulence rises. A modification to Hancock’s [1] empirical correlation relating the fractional increase in skin friction at constant Reynolds number to a free stream turbulence parameter containing a dependence on both intensity and length scale is proposed. While this modification has the necessary characteristic of being a function of the free stream turbulence parameters as well as the Reynolds number, it is argued that the relative importance of intensity and length scale changes at low Reynolds numbers; the data are not inconsistent with this idea. The experiments cover the range 500 ⪝ Reθ ⪝ 2500, u′/Ue ⪝ 0.07, 0.8 ⪝ Le/δ ⪝ 2.9, where u′/Ue is the free stream turbulence intensity and Le/δ is the ratio of the dissipation length scale of the free stream turbulence to the 99 percent thickness of the boundary layer.

Flow past a transversely oscillating square cylinder in free stream at low Reynolds numbers

2009 ◽  
Vol 61 (6) ◽  
pp. 658-682 ◽  
Author(s):  
A. P. Singh ◽  
A. K. De ◽  
V. K. Carpenter ◽  
V. Eswaran ◽  
K. Muralidhar
Keyword(s):  
Free Stream ◽  
Reynolds Numbers ◽  
Square Cylinder ◽  
Low Reynolds Numbers ◽  
Flow Past ◽  
Low Reynolds ◽  
Oscillating Square Cylinder

Effect of Free-Stream Turbulence on Heat Transfer through a Turbulent Boundary Layer

1978 ◽  
Vol 100 (4) ◽  
pp. 671-677 ◽  
Author(s):  
J. C. Simonich ◽  
P. Bradshaw
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Reynolds Number ◽  
Free Stream ◽  
Reynolds Numbers ◽  
Skin Friction Coefficient ◽  
Reynolds Analogy ◽  
Low Reynolds Numbers ◽  
Free Stream Turbulence ◽  
High Reynolds

Measurements in a boundary layer in zero pressure gradient show that the effect of grid-generated free-stream turbulence is to increase heat transfer by about five percent for each one percent rms increase of the longitudinal intensity. In fact, even a Reynolds analogy factor, 2 × (Stanton number)/(skin-friction coefficient), increases significantly. It is suggested that the irreconcilable differences between previous measurements are attributable mainly to the low Reynolds numbers of most of those measurements. The present measurements attained a momentum-thickness Reynolds number of 6500 (chord Reynolds number approximately 6.3 × 106) and are thought to be typical of high-Reynolds-number flows.

scholarly journals Validation of a CFD Methodology for Variable Speed Power Turbine Relevant Conditions

2013 ◽  
Author(s):  
Ali A. Ameri ◽  
Paul W. Giel ◽  
Ashlie B. McVetta
Keyword(s):  
Three Dimensional ◽  
Reynolds Numbers ◽  
Transitional Flow ◽  
Variable Speed ◽  
Low Reynolds Numbers ◽  
Pressure Losses ◽  
Pressure Distributions ◽  
Power Turbine ◽  
Wide Range ◽  
Transfer Problems

Analysis tools are needed to investigate aerodynamic performance of Variable-Speed Power Turbines (VSPT) for rotorcraft applications. The VSPT operates at low Reynolds numbers (transitional flow) and over a wide range of incidence. Previously, the capabilities of a published three-equation transition and turbulence model in predicting the transition location for three-dimensional heat transfer problems were assessed. In this paper, results are presented of a post-diction exercise using a three-dimensional flow in a transonic linear cascade comprising VSPT blading. The measured pressure distributions and integrated spanwise total pressure losses and flow angles for two incidence angles corresponding to cruise (i = +5.8°) and takeoff (i = –36.7°) were used for this study. For the higher loading condition of cruise and the negative incidence condition of takeoff, overall agreement with data may be considered satisfactory but areas of needed improvement are also indicated.

Investigation on the effect of leading edge tubercles of sweptback wing at low reynolds number

2020 ◽  
Vol 21 (6) ◽  
pp. 621
Author(s):  
Veerapathiran Thangaraj Gopinathan ◽  
John Bruce Ralphin Rose ◽  
Mohanram Surya
Keyword(s):  
Leading Edge ◽  
Reynolds Numbers ◽  
Sweep Angle ◽  
Laminar Separation ◽  
Low Reynolds Numbers ◽  
Flow Energy ◽  
Airplane Wing ◽  
Low Reynolds ◽  
Naca 0015 ◽  
Wing Performance

Aerodynamic efficiency of an airplane wing can be improved either by increasing its lift generation tendency or by reducing the drag. Recently, Bio-inspired designs have been received greater attention for the geometric modifications of airplane wings. One of the bio-inspired designs contains sinusoidal Humpback Whale (HW) tubercles, i.e., protuberances exist at the wing leading edge (LE). The tubercles have excellent flow control characteristics at low Reynolds numbers. The present work describes about the effect of tubercles on swept back wing performance at various Angle of Attack (AoA). NACA 0015 and NACA 4415 airfoils are used for swept back wing design with sweep angle about 30°. The modified wings (HUMP 0015 A, HUMP 0015 B, HUMP 4415 A, HUMP 4415 B) are designed with two amplitude to wavelength ratios (η) of 0.1 & 0.24 for the performance analysis. It is a novel effort to analyze the tubercle vortices along the span that induce additional flow energy especially, behind the tubercles peak and trough region. Subsequently, Co-efficient of Lift (CL), Co-efficient of Drag (CD) and boundary layer pressure gradients also predicted for modified and baseline (smooth LE) models in the pre & post-stall regimes. It was observed that the tubercles increase the performance of swept back wings by the enhanced CL/CD ratio in the pre-stall AoA region. Interestingly, the flow separation region behind the centerline of tubercles and formation of Laminar Separation Bubbles (LSB) were asymmetric because of the sweep.

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