Flat Plate Test Research Articles

Transition Prediction on Turbine Blade Profile With Intermittency Transport Equation

This paper presents the results of tests and validations of the γ-Reθ model proposed by Menter et al. (2006, “A Correlation-Based Transition Model Using Local Variables—Part I: Model Formation,” ASME J. Turbomach., 128, pp. 413–422), which was extended by in-house correlations for onset location and transition length. The tests performed were based on experimental data from the flat plate test cases available at the ERCOFTAC database as well as on experimental data from the turbine blade profile investigated at Czestochowa University of Technology. Further on, the model was applied for unsteady calculations of the blade profile test case, where chosen inlet conditions (turbulent intensity and wake parameters) were applied. For the selected case, numerical results were compared not only with the experimental data but also with the results obtained with other transition models. It was shown that the applied model was able to reproduce some essential flow features related to the bypass and wake-induced transition, and the simulations revealed good agreement with the experimental results in terms of localization and extent of wake-induced transition.

Development of a Modified Flat-plate Test Specimen and Fixture for Composite Materials Crush Energy Absorption

There are currently no specialized test methods for the characterization of specific energy absorption (SEA) of composite materials. Based on an original concept developed by NASA in the early 1990s, a test method that utilizes a flat plate-like specimen and a modified anti-buckling fixture is presented here that introduces a region of unsupported material in the proximity of the crush front, and allows the specimen to deform freely. A systematic experimental investigation is conducted with two unidirectional tapes to verify the general applicability of the test method to screen candidate material systems, and to isolate the effect of the unsupported distance on the measured SEA values. In general, it is found that there are four failure mechanisms (fragmentation, frond formation, local, and global buckling) that compete as the dominating mode according to the chosen unsupported distance. The specimen and fixture combination presents several limitations, but with a properly selected unsupported distance it could be used to assess the SEA of material and structures whose dominating failure mechanism is frond formation.

Assessment of Gas-Surface Interaction Models for Computation of Rarefied Hypersonic Flow

conditions, although various models have been developed. The goal of this study is the assessment of gas–surface interaction models in use with the direct simulation Monte Carlo method. Assessment is made of the two most common gas–surface interaction models in use with direct simulation Monte Carlo: the Maxwell model and the Cercignani, Lampis, and Lord model. The assessment is performed by simulations of flat-plate wind-tunnel tests. Boundary-layer profiles are compared with existing wind-tunnel data. At about 90% accommodation, both models match the wind-tunnel profiles. Parametric studies demonstrate differences between the model predictions of scattering distributions, boundary-layer profiles, and surface-property distributions. The two models offer similar performance for computing flat-plate aerodynamics.

Numerical Simulation of Bypass Transition by the Approach of Intermittency Transport Equation

This paper deals with the development of a numerical approach to predict freestream turbulence-induced (FST-induced) boundary layer bypass transition using an intermittency transport equation. An intermittency based transition model, which is critical for invoking transition onset according to Abu-Ghannam and Shaw correlation, is implemented into the proven Reynolds-Averaged N-S (RANS) solver. The intermittent behavior of the transitional flow is incorporated into the computation by modifying the eddy viscosity t µ , obtained from a turbulence model. Wilcox low Reynolds k ω − turbulence model is employed to calculate the eddy viscosity and others turbulent quantities. For validation, the present transition model is applied to the benchmark experiments of flat plate test cases of ERCOFTAC series and to predictions of a modern Low Pressure (LP) turbine flow. It follows from the detail comparisons of the calculated results with the relevant experimental data and other researchers’ simulations that the present model is capable to make a reasonable prediction of FST-induced bypass transition.

A two‐algebraic turbulence model for compressible flow in turbomachinery cascade

AbstractThe objective of this study is to examine the Baldwin–Lomax turbulence model in a finite volume solver to introduce a computer code for complex two–dimensional flows in turbomachinery. The turbulent model was tested with investigating the turbulent flow over a flat plate and other test cases. Three test cases are presented and the computed results are compared with experimental data. The calculated velocity profile agreed well with the experimental data in plate test case and the solver is validated in test case of flow over a semi NACA–0012 airfoil. The solver is used for flow through a multi–blade cascade of an axial compressor in design condition to show its capability of multi–block solution. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Bubble-induced skin-friction drag reduction and the abrupt transition to air-layer drag reduction

To investigate the phenomena of skin-friction drag reduction in a turbulent boundary layer (TBL) at large scales and high Reynolds numbers, a set of experiments has been conducted at the US Navy's William B. Morgan Large Cavitation Channel (LCC). Drag reduction was achieved by injecting gas (air) from a line source through the wall of a nearly zero-pressure-gradient TBL that formed on a flat-plate test model that was either hydraulically smooth or fully rough. Two distinct drag-reduction phenomena were investigated; bubble drag reduction (BDR) and air-layer drag reduction (ALDR).The streamwise distribution of skin-friction drag reduction was monitored with six skin-friction balances at downstream-distance-based Reynolds numbers to 220 million and at test speeds to 20.0ms−1. Near-wall bulk void fraction was measured at twelve streamwise locations with impedance probes, and near-wall (0 <Y< 5mm) bubble populations were estimated with a bubble imaging system. The instrument suite was used to investigate the scaling of BDR and the requirements necessary to achieve ALDR.Results from the BDR experiments indicate that: significant drag reduction (>25%) is limited to the first few metres downstream of injection; marginal improvement was possible with a porous-plate versus an open-slot injector design; BDR has negligible sensitivity to surface tension; bubble size is independent of surface tension downstream of injection; BDR is insensitive to boundary-layer thickness at the injection location; and no synergetic effect is observed with compound injection. Using these data, previous BDR scaling methods are investigated, but data collapse is observed only with the ‘initial zone’ scaling, which provides little information on downstream persistence of BDR.ALDR was investigated with a series of experiments that included a slow increase in the volumetric flux of air injected at free-stream speeds to 15.3ms−1. These results indicated that there are three distinct regions associated with drag reduction with air injection: Region I, BDR; Region II, transition between BDR and ALDR; and Region III, ALDR. In addition, once ALDR was established: friction drag reduction in excess of 80% was observed over the entire smooth model for speeds to 15.3ms−1; the critical volumetric flux of air required to achieve ALDR was observed to be approximately proportional to the square of the free-stream speed; slightly higher injection rates were required for ALDR if the surface tension was decreased; stable air layers were formed at free-stream speeds to 12.5ms−1with the surface fully roughened (though approximately 50% greater volumetric air flux was required); and ALDR was sensitive to the inflow conditions. The sensitivity to the inflow conditions can be mitigated by employing a small faired step (10mm height in the experiment) that helps to create a fixed separation line.

Experimental and CFD Analyses Examining Ozone Distribution in Model Rooms with Laminar and Turbulent Flow Fields

The principle goal of this work was to better understand ozone distribution within rooms. Towards this end, the paper has two parts. The first describes the development of a flat plate test chamber (FPT chamber) that can be used to obtain mass accommodation coefficients (γ) for ozone that deposits on the surface of different materials. The second consists of model room experiments coupled with Computational Fluid Dynamics (CFD) analysis of the experimental scenarios to obtain ozone distribution in turbulent flow fields.

Enhanced energy dissipation rates in laminar boundary layers subjected to elevated levels of freestream turbulence

Enhanced energy dissipation rates in laminar boundary layers subjected to elevated levels of freestream turbulence are investigated for zero pressure gradient flow with freestream turbulence intensities ranging from 0.2% to 7%. The freestream turbulence markedly changes both the mean and fluctuating velocity distributions resulting in increased energy dissipation rates per unit area by up to 34%. A shortcoming of current numerical and analytical techniques is the inability to accurately predict this increased energy dissipation. A new correlation, based upon experimental measurements using the hotwire technique, has been developed to account for this increased rate of energy dissipation. The correlation developed implements the momentum thickness Reynolds number and turbulence intensity at the leading edge to capture the enhanced energy dissipation rates due to elevated freestream turbulence intensity in the laminar boundary layers investigated. The correlation is then applied to well-known flat plate test cases and good agreement is found.

Application of the Turbulent Potential Model to Heat Transfer Predictions on a Turbine Guide Vane

The accurate numerical simulation of the flow through turbomachinery depends on the reliable prediction of laminar to turbulent boundary layer transition phenomena. The aim of this paper is to study the ability of the turbulent potential model to predict those nonequilibrium turbulent flows for several test cases. Within this model turbulent quantities are described by the turbulent scalar and turbulent vector potentials of the turbulent body force—the divergence of the Reynolds stress tensor. For model validation first flat plate test cases with different inlet turbulence intensities, zero pressure gradient, and nonuniform pressure gradient distributions along the plate were calculated and compared by means of skin friction values measured in the experiments. Finally the model was validated by heat transfer measurement data obtained from a highly loaded transonic turbine guide vane cascade for different operating conditions.

Intermittency based RANS bypass transition modelling

A transition model for describing bypass transition is presented. It is based on a two-equations k-ω model and a dynamic equation for intermittency factor. This intermittency factor is a multiplier of the turbulent viscosity computed by the turbulence model. Following a suggestion by Menter et al. (2002), the start of transition is computed based on local variables. The quality of the transition model, developed on flat plate test cases is illustrated on turbine cascades.

Evaluation of Near-Wall Turbulence Models for Deliberately Roughened Liquid Annular Seals

An extensive numerical evaluation of the performance of two near-wall treatments and two core-flow turbulence models has been done while simulating experimental tests of water flowing between flat plates with deliberately roughened surfaces. These flat-plate tests replicated the results of several annular seal experiments in which friction factor increased as the seal clearance increased. Annular seals are extensively used in a wide range of turbomachinery, and their design influences efficiency and rotordynamic stability. A commercial code, FLUENT, was used to solve the Reynolds-averaged Navier‐Stokes equations with the flat-plate tests as a reference. The performance of the standard wall functions, based on the law of the wall, as near-wall treatment of turbulence was evaluated and compared to the two-layer-zonal approach. Similarly, the solutions obtained with the standard and “renormalization group” method versions of the first-order closure k‐e model were contrasted to those obtained with a second-order closure, the Reynolds-stress model. Although the main features of the friction factor behavior observed in the experiments under study were reproduced with a simple two-dimensional approach, the present work concentrates on the numerical detail and lessons learned while obtaining such results. It was found that wall function solutions are extremely sensitive to the location of the first grid point near the wall, even if it is located within the overlap region. In addition, the low-Reynolds nature of the flow requires coarse meshing making any k‐e model solution grid dependent. On the other hand, the two-layer zonal model is more consistent, and it is not sensitive to the location of the first grid point near the wall, provided it is located at a y + distance no larger than 10. Solutions obtained with the latter approach and the Reynolds-stress model are found to be grid independent. In summary, a numerical solution is truly grid independent when it is consistently replicated upon variation of the discretization scheme, the pressure-to-velocity linking method, the algebraic equation solving algorithm, and the mesh size and type. In low-Reynolds-number turbulent flow, the two-layer zonal approach fulfills such statement as near-wall treatment of turbulence.

Film cooling characteristics of rows of round holes at various streamwise angles in a crossflow: Part I. Effectiveness

Film cooling effectiveness were studied experimentally on rows of cylindrical holes with streamwise angles of 30°, 60° and 90°, in a flat plate test facility with a zero pressure gradient. Detailed effectiveness and heat transfer results for a single cylindrical hole at the same inclinations have been presented in Yuen and Martinez-Botas [C.H.N. Yuen, R.F. Martinez-Botas, Film cooling characteristics of a single hole at various streamwise angles: Part I. Effectiveness, Int. J. Heat Mass Transfer 46 (2003) 221–235; C.H.N. Yuen, R.F. Martinez-Botas, Film cooling characteristics of a single hole at various streamwise angles: Part II. Heat transfer coefficient, Int. J. Heat Mass Transfer 46 (2003) 237–249] with the same test facility and measurement technique. This present investigation commenced with a single row of holes with two pitch-to-diameter ratios ( p/D) of 3 and 6. It then presents and discusses the effects of introducing inline and staggered rows for each streamwise angle and pitch-to-diameter ratio. The row spacing in the inline and staggered rows is 12.5 diameters in the streamwise direction. The short but engine representative hole length ( L/D = 4) is constant for all geometries. The blowing ratio ranges from 0.33 to 2, and the freestream Reynolds number based on the freestream velocity and hole diameter ( Re D) was 8563. Both local values and laterally averaged ones are presented, the latter refers to the averaged value across the central hole. The current results are compared with the experimental results obtained by other researchers, the effects of the additional inline and staggered rows, and of the variations in injection angle, pitch-to-diameter ratio are described. The objectives of the present study are to provide a consistent set of measurements in terms of effectiveness and heat transfer coefficients presented in the companion paper [C.H.N. Yuen, R.F. Martinez-Botas, Film cooling characteristics of rows of round holes at various streamwise angles: Part II. Heat transfer coefficient, Int. J. Heat Mass Transfer, in press], obtained systematically with the same test facility, and to deliver a better understanding of film cooling performance. The present results also serve as a database with 105 test cases, in addition to the 21 cases presented in [C.H.N. Yuen, R.F. Martinez-Botas, Film cooling characteristics of a single hole at various streamwise angles: Part I. Effectiveness, Int. J. Heat Mass Transfer 46 (2003) 221–235], for future numerical modelling.

Demonstration of skin friction measurements featuring in situ estimation of conduction loss using constant voltage anemometers and surface hot-films

The top of the 12.2m long NASA Dryden Flight Research Center’s ground research vehicle (GRV) was used as a flat plate test bed for demonstrating an approach to measure skin friction. Using an array of surface hot-films operated by constant voltage anemometers (CVAs), the approach was demonstrated with in situ estimation of conduction heat loss from the hot-films to the substrate. An algebraic relationship, using the channel calibration constants a and b (determined a priori) with CVA output voltages Vs and Vw from that channel, is used for the estimation of the required quantities and lead resistance (rL) of the hot-film measured on site. Estimates of the power dissipated in the hot-film alone (Phf) (excluding the lead resistances), in situ resistance (Rw) of the hot-film due to applied overheat and flow, and the cold resistance (Ra) of the same hot-film at the ambient temperature are so obtained. Different approaches to estimate the in situ cold resistance (which is the resistance without any self-heating) of the hot-film are presented addressing the suitability of the procedure for flight applications as well. Tests were performed at several speeds of the GRV on the tarmac of a runway at the flight test center. The measured values are fitted to the classical (13) law equation with the computational dimensional skin friction (τ) obtained using the empirical local skin friction law for the long flat plate. There was an excellent (1∕3) law fit in all the hot-films, demonstrating that the measured values fit classical theory. Using this measured fit with the theoretical values, calibration coefficients (A and B) for dimensional skin friction (τ) were obtained. Using these calibration coefficients, measured values were then converted to nondimensional local skin friction coefficients cf for all the hot-films at all speeds. Measured cf values agree well with the associated flat plate theory. Since the in situ measurement of heat loss to the substrate should ideally apply at all ambient temperatures, the method was tested in a wind tunnel at two ambient temperatures, where the results repeated well. Accounting for conduction loss had been the difficult issue in using hot-films, which otherwise offer many advantages. For example, skin friction from an array of hot-films can be measured simultaneously and the hot-films offer large dynamic response, high sensitivity, and simple installation. In practice, it should be possible to insert a calibrated plug in the test article and use the in situ measured substrate loss to accurately estimate the skin friction. The approach does not need any additional temperature sensor to accomplish the measurements. The system is highly amenable to automation with further work. It may be noted that the purpose of this test and article is to demonstrate an approach to skin friction measurements using hot-films operated by CVAs with in situ estimation of the heat loss to the substrate. It does not address any other aspect of the skin friction measurements.

Frictional Resistance of Antifouling Coating Systems

An experimental study has been made to compare the frictional resistance of several ship hull coatings in the unfouled, fouled, and cleaned conditions. Hydrodynamic tests were completed in a towing tank using a flat plate test fixture towed at a Reynolds number ReL range of 2.8×106-5.5×106 based on the plate length and towing velocity. The results indicate little difference in frictional resistance coefficient CF among the coatings in the unfouled condition. Significant differences were observed after 287 days of marine exposure, with the silicone antifouling coatings showing the largest increases in CF. While several of the surfaces returned to near their unfouled resistance after cleaning, coating damage led to significant increases in CF for other coatings. The roughness function ΔU+ for the unfouled coatings showed reasonable collapse to a Colebrook-type roughness function when the centerline average height k=0.17Ra was used as the roughness length scale. Excellent collapse of the roughness function for the barnacle fouled surfaces was obtained using a new roughness length scale based on the barnacle height and percent coverage.

Test results of friction factor for round-hole roughness surfaces in closely spaced channel flow of water

For examining friction-factor characteristics of round-hole pattern surfaces which are usually applied on damper seals, flat plate test apparatus is designed and fabricated. The measurement method of leakage and pressure distribution along round-hole pattern specimen with different hole area is described and a method for determining the Fanning friction factor is discussed. Results show that the round-hole pattern surfaces provide a much larger friction factor than smooth surface, and the friction factor vs. clearance behavior yields that the friction factor generally decreases as the clearance increases unlike the results of Nava’s flat plate test. As the hole depth is decreased, the friction factor is increased, and maximum friction factor is obtained for 50% of hole area. Since the present experimental friction factor results show coincident characteristics with Moody’s friction factor model, empirical friction factors for round-hole pattern surfaces are obtained by using the Moody’s formula based on curve-fit of the experimental data. Results of Villasmil’s 2D CFD simulation support the present experimental test result.

Intermittency Transport Modeling of Separated Flow Transition

An intermittency transport model is proposed for modeling separated-flow transition. The model is based on earlier work on prediction of attached flow bypass transition and is applied for the first time to model transition in a separation bubble at various degrees of freestream turbulence. The model has been developed so that it takes into account the entrainment of the surrounding fluid. Experimental investigations suggest that it is this phenomena which ultimately determines the extent of the separation bubble. Transition onset is determined via a boundary layer correlation based on momentum thickness at the point of separation. The intermittent flow characteristic of the transition process is modeled via an intermittency transport equation. This accounts for both normal and streamwise variation of intermittency and hence models the entrainment of surrounding flow in a more accurate manner than alternative prescribed intermittency models. The model has been validated against the well-established T3L semicircular leading edge flat plate test case for three different degrees of freestream turbulence characteristic of turbomachinery blade applications.

임의로 거칠게 한 표면의 점성 마찰특성

For measuring friction factor of artificially-roughened surfaces which are usually applied to damper seals, flat plate test apparatus is designed and fabricated. The measurements of leakage flow and pressure distribution through round-hole patterned specimen with different hole areas are described, and a method is discussed for determining the friction factor experimentally. Results show that the friction factor of the round-hole patterned surface is bigger than that of smooth surface, and increases as increasing the hole area. A empirical friction factor model for the round-hole patterned surface can be descrived by the Moody`s friction factor formula.

Film cooling characteristics of a single round hole at various streamwise angles in a crossflow: Part I effectiveness

Film cooling effectiveness were studied experimentally on a cylindrical hole with a streamwise angle of 30°, 60° and 90°, in a flat plate test facility with a zero pressure gradient. The short but engine representative hole length ( L/ D=4) is constant for all three geometries. The blowing ratio ranges from 0.33 to 2, and the freestream Reynolds number based on the freestream velocity and hole diameter ( Re D) was 8563. Both local values and laterally averaged ones are presented, the latter refers to the averaged value across the central hole. The current results are compared with the experimental results obtained by other researchers, and the effects of variations in injection angle are described. There is not a single study on film cooling performance of a single round hole at the three streamwise angles for a wide range of blowing ratios with a short hole length. Therefore, the objective of the present study is to provide a consistent set of measurements in terms of effectiveness and heat transfer coefficients [Int. J. Heat Mass Transfer, 2002] obtained systematically on a flat plate, and to deliver a better understanding of film cooling performance, with explanations deduced from the existing knowledge of a single jet in crossflow, before more engine representative conditions are introduced. The present results also serve as a database for future numerical modelling.

A Converging Slot-Hole Film-Cooling Geometry—Part 2: Transonic Nozzle Guide Vane Heat Transfer and Loss

This paper presents the first experimental measurements on an engine representative nozzle guide vane, of a new film-cooling hole geometry, a con¯vergings¯lot-hole¯ or console. The patented console geometry is designed to improve the heat transfer and aerodynamic performance of turbine vane and rotor blade cooling systems. These experiments follow the successful validation of the console design in low-speed flat-plate tests described in Part 1 of this paper. Stereolithography was used to manufacture a resin model of a transonic, engine representative nozzle guide vane in which seven rows of previously tested fan-shaped film-cooling holes were replaced by four rows of consoles. This vane was mounted in the annular vane ring of the Oxford cold heat transfer tunnel for testing at engine Reynolds numbers, Mach numbers and coolant to mainstream momentum flux ratios using a heavy gas to simulate the correct coolant to mainstream density ratio. Heat transfer data were measured using wide-band thermochromic liquid crystals and a modified analysis technique. Both surface heat transfer coefficient and the adiabatic cooling effectiveness were derived from computer-video records of hue changes during the transient tunnel run. The cooling performance, quantified by the heat flux at engine temperature levels, of the console vane compares favourably with that of the previously tested vane with fan-shaped holes. The new console film-cooling hole geometry offers advantages to the engine designer due to a superior aerodynamic efficiency over the fan-shaped hole geometry. These efficiency measurements are demonstrated by results from midspan traverses of a four-hole pyramid probe downstream of the nozzle guide vane.

The Relationship Between Frictional Resistance and Roughness for Surfaces Smoothed by Sanding

An experimental investigation has been carried out to document and relate the frictional resistance and roughness texture of painted surfaces smoothed by sanding. Hydrodynamic tests were carried out in a towing tank using a flat plate test fixture towed at a Reynolds number ReL range of 2.8×106−5.5×106 based on the plate length and freestream velocity. Results indicate an increase in frictional resistance coefficient CF of up to 7.3% for an unsanded, as-sprayed paint surface compared to a sanded, polished surface. Significant increases in CF were also noted on surfaces sanded with sandpaper as fine as 600-grit as compared to the polished surface. The results show that, for the present surfaces, the centerline average height Ra is sufficient to explain a large majority of the variance in the roughness function ΔU+ in this Reynolds number range.