Investigation of the law-of-the-wall for a turbulent boundary layer flow subject to an adverse pressure gradient using multi-resolution particle imaging
Tobias KnoppDaniel SchanzAndreas SchröderNicolas BuchmannChristian CierpkaRainer HainChristian J. Kähler
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Abstract:
The reliable prediction of low-speed flow separation of a turbulent boundary layer on a smooth surface due to an adverse pressure gradient is still an open issue. In the literature, there is no agreement on the law-of-the-wall for adverse pressure gradients. The objective of a recent and ongoing research investigation started in the DLR internal project RETTINA is to build up a worldwide unique set of experimental data for a turbulent boundary layer flow at adverse pressure gradient at high Reynolds numbers by a joint PIV campaign of DLR and UniBw Munchen, and to investigate existing and to develop improved proposals for the law-of-the-wall in case of an adverse pressure gradient.Keywords:
Adverse pressure gradient
Pressure gradient
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The requirement for reduced jet noise in order to meet stringent noise legislation (civil
aviation), and low infra-red observability and the use of unconventional exhaust nozzle
configurations to improve aircraft survivability and performance (military aviation) is driving
research to develop a better understanding of jet development and mixing mechanisms. One
option open to the engineer is the use of small-scale model testing to investigate jets flows
and provide valuable data for the validation of numerical models. Although more economical
than large/full scale testing, additional factors that influence jet development may be present
which would not be present at full scale and whose influence needs to be fully understood
in order to allow small scale–large scale read-across. One such factor is the nozzle exit
boundary layer. Although considerable data exist on the influence of nozzle exit boundary
layers on low speed jet flows, current information on high speed jet flows is limited. It
was, therefore, the aim of this thesis to extend the current understanding of nozzle exit
boundary layers and their influence on the jet development for high speed jet flows through
a combination of experimental and computational techniques.
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thickness Reynolds number and changed the state of the boundary layer from turbulent to
laminar-like. The addition of a parallel extension to the nozzle exit returned the boundary
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to become fully turbulent.
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Excessive base heating has been a problem for many launch vehicles. For certain designs such as the direct dump of turbine exhaust inside and at the lip of the nozzle, the potential burning of the turbine exhaust in the base region can be of great concern. Accurate prediction of the base environment at altitudes is therefore very important during the vehicle design phase. Otherwise, undesirable consequences may occur. In this study, the turbulent base flowfield of a cold flow experimental investigation for a four-engine clustered nozzle was numerically benchmarked using a pressure-based computational fluid dynamics (CFD) method. This is a necessary step before the benchmarking of hot flow and combustion flow tests can be considered. Since the medium was unheated air, reasonable prediction of the base pressure distribution at high altitude was the main goal. Several physical phenomena pertaining to the multiengine clustered nozzle base flow physics were deduced from the analysis.
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Herein, we describe the design and testing of a stereoscopic PIV system uniquely adapted for the high pressure environment of the Princeton Superpipe. The Superpipe is a recirculating pipe facility that utilizes compressed air as the working fluid to attain very high Reynolds numbers. Commercial piping is used as the pressure vessel to hold pressure up to 220 bars, and a test pipe is enclosed inside with a development length of 200 diameters that ensures a fully-developed condition at the test section. The highest achievable Reynolds number (based on the bulk velocity and the pipe diameter) is 35×106, corresponding to a maximum friction Reynolds number of 5×105.
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ABSTRACT The general picture of research in active flow control for aircraft applications has been continuously changing over the last 20 years. Researchers can now obtain design sensitivities by using numerical flow simulations, and new optical experimental methods can be used that measure flow field data non-intrusively in planes and volumes. These methodological advances enabled significant knowledge increase. The present paper reviews recent progress in active flow control by steady blowing. It appears that two strategies of blowing deserve particular attention. The first uses tangential blowing of thin wall jets to overcome the adverse pressure gradients from locally very large flow turning rates. This approach exploits the potentials of the Coanda effect. The second strategy employs oblique blowing of air jets designed to generate longitudinal vortices in the boundary layer. The longitudinal vortices provide convective redistribution of momentum in the boundary layer, and they also enhance turbulent momentum transport. The sensitivities of these two approaches as observed in fundamental flow investigations and in applications to high-lift aerofoils are described and suited efficiency parameters of blowing are analysed.
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An experimental facility specifically designed to investigate internal fluid duct flows is described. It is built in a modular fashion so that a variety of internal flow test hardware can be installed in the facility with minimal facility reconfiguration. The facility and test hardware interfaces are discussed along with design constraints of future test hardware. The plenum flow conditioning approach is also detailed. Available instrumentation and data acquisition capabilities are discussed. The incoming flow quality was documented over the current facility operating range. The incoming flow produces well behaved turbulent boundary layers with a uniform core. For the calibration duct used, the boundary layers approached 10 percent of the duct radius. Freestream turbulence levels at the various operating conditions varied from 0.64 to 0.69 percent of the average freestream velocity.
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Internal flow
Freestream
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Investigations were performed to develop accurate boundary-layer measurement techniques in a Mach 3.5 laminar boundary layer on a 7 half-angle cone at 0 angle of attack. A discussion of the measurement challenges is presented as well as how each was addressed. A computational study was performed to minimize the probe aerodynamic interference effects resulting in improved pitot and hot-wire probe designs. Probe calibration and positioning processes were also developed with the goal of reducing the measurement uncertainties from 10% levels to less than 5% levels. Efforts were made to define the experimental boundary conditions for the cone flow so comparisons could be made with a set of companion computational simulations. The development status of the mean and dynamic boundary-layer flow measurements for a nominally sharp cone in a low-disturbance supersonic flow is presented.
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The objective of this study is to gain insight into the complex flow phenomena, governing the aerodynamics of intakes for airbreathing space vehicles. Complementary to CFD-simulations, reliable wind tunnel experiments remain necessary for validation as well as for understanding physical effects and their impact on the overall design. The hypersonic blow down tunnel H2K with its long test duration ensures a fully established flow field to simulate external and internal compression processes of different inlet models with three ramps.
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This work deals with the documentation and control of flow separation that occurs over turbine blades in the low-pressure turbine stage at low Reynolds numbers that exist at high altitude cruise. We utilize a specially constructed linear cascade that is designed to study the flow field over a generic LPT cascade consisting of Pratt & Whitney 'Pak B' shaped blades. This facility was constructed under a previous one-year NASA Glenn RC initiative. The center blade in the cascade is instrumented to measure the surface pressure coefficient distribution. Optical access allows two-component LDV measurement for boundary layer profiles. Experimental conditions have been chosen to give a range of chord Reynolds numbers from 10 to 100K, and a range of free-stream turbulence levels from u'/U(sub infinity)= 0.08 to 3 percent. The surface pressure measurements were used to define a region of separation and reattachment that depend on the free-stream conditions. The location of separation was found to be relatively insensitive to the experimental conditions. However, reattachment location was very sensitive to the turbulence level and Reynolds number. Excellent agreement was found between the measured pressure distributions and predictions from Euler and RANS simulations. Two-component LDV measurements are presently underway to document the mean and fluctuating velocity components in the boundary layer over the center blade for the range of experimental conditions. The fabrication of the plasma actuator is underway. These are designed to produce either streamwise vortices, or a downstream-directed wall jet. A precursor experiment for the former approach was performed with an array of vortex generators placed just upstream of the separation line. These led to reattachment except for the lowest Reynolds number. Progress has also been made on the proposed concept for a laterally moving wake. This involved constructing a smaller wind tunnel and molding an array of symmetric airfoils to form an array. Following its development, it will be scaled up and used to introduce lateral moving wakes upstream up the Pak-B cascade.
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The present paper describes the application of time-resolved tomographic PIV [1, 2] to a turbulent boundary layer flow influenced by vortex generators. Flow control is a promising means to increase the performance of aerodynamic systems. Both active (e.g. suction) and passive (e.g. vortex generator vanes) devices have successfully been used in aerodynamic research as well as commercial application. One big area of application is the suppression or at least delay of separation of the flow around airfoils at high angles of attack. As a general rule passive devices have the advantage of being cost-effective and simple to setup; Successful examples to this are vortex generator vanes [3]. The principle of operation is based on the increase of momentum exchange from the free flow into the boundary layer. For a deeper understanding and the optimization of these mechanisms, three-dimensional measurements of the flow can provide valuable information. Instantaneous recordings of the complete volume can serve as a data basis for numerical simulations, especially if the data is also time-resolved.
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