Flow Conditions In Wind Tunnel Research Articles

Global stability analysis of elastic aircraft in edge-of-the-envelope flow

Shock buffet on wings is a phenomenon caused by strong shock-wave/boundary-layer interaction resulting first in self-sustained flow unsteadiness and eventually in a detrimental structural response called buffeting. While it is an important aspect of wing design and aircraft certification, particularly for modern transonic air transport, not all of the underlying multidisciplinary physics is thoroughly understood. Building upon a single-discipline shock-buffet stability study, this work now investigates the impact of an elastic structure in these extreme flow conditions. Specifically, a triglobal stability analysis of a fluid–structure coupled system is presented, utilising the implicitly restarted Arnoldi method with a sparse iterative Krylov solver and novel preconditioner. Asymmetry resulting from a static aeroelastic simulation based on a finite-element model of the underlying geometry in a wind tunnel modifies the global modes of the earlier fluid-only symmetric full-span analysis. A flutter stability analysis at wind-tunnel flow conditions below shock-buffet onset finds no instability in the structural degrees-of-freedom, whereas in shock-buffet flow with globally unstable fluid modes additional marginally unstable structural (and fluid) modes emerge. The developed stability tool for coupled analysis is instrumental in identifying those physically relevant and strongly coupled modes where a standard pk-type (p being eigenvalue and k reduced frequency) flutter analysis fails. With the complementary computation of adjoint eigenmodes, the core of the instability is pinpointed to a relatively small wing area which may help to effect the control and delay of this detrimental transonic unsteadiness. We contribute to the question on how the presence of the elastic wing structure impacts on the otherwise pure aerodynamic three-dimensional shock-buffet dynamics.

Modal decomposition of the pressure field on a bridge deck under vortex shedding using POD, DMD and ERA with correlation functions as Markov parameters

A pressure field on a stationary bridge section with a clear signature from vortex shedding is obtained from a numerical simulation. The pressure field is subjected to modal decomposition using the snapshot prober orthogonal decomposition method (POD), the exact dynamic mode decomposition method (DMD) and a novel application of the eigenvalue realization algorithm (ERA) where the correlation functions of the pressure field is used as Markov parameters. All three methods are able to identify spatial mode shapes of the surface pressure field due to vortex shedding. The Strouhal number is estimated using dynamic mode decomposition and the eigenvalue realization algorithm where the latter is the most accurate and precise method. Finally, the ERA is applied to estimate Strouhal numbers and vortex shedding pressure mode shapes under different flow conditions in wind tunnel and full-scale testing of the Gjemnessund bridge in Norway.

Instability and transition mechanisms induced by skewed roughness elements in a high-speed laminar boundary layer

The disturbance evolution in a Mach-4.8 zero-pressure-gradient flat-plate boundary-layer flow altered by discrete three-dimensional roughness elements is investigated including a laminar breakdown scenario. Direct numerical simulation (DNS), as well as the biglobal linear stability theory based on two-dimensional eigenfunctions in flow cross-sections, are applied. Roughness elements with high ratios of spanwise width to streamwise length are compared at varying height and skewing angles with respect to the oncoming flow. For an oblique roughness, the element’s height is varied between 27 % and 68 % of the undisturbed boundary-layer thickness. Compared to a symmetric roughness element an obliquely placed element generates a more pronounced low-speed streak in the roughness wake. The linear stability analysis reveals the occurrence of eigenmodes that can be associated with the first and second modes in the flat-plate flow. At identical roughness height, larger amplification is found for the eigenmodes of the oblique set-up. The results are confirmed by unsteady DNS showing very good agreement with stability theory; transient-growth behaviour in the near wake of the roughness is of minor importance. The comparison of the results gained for adiabatic wind-tunnel flow conditions with those for atmospheric-flight conditions with wall cooling reveals significant differences in the wake vortex system with subsequent impact on the stability properties of the flow. The hot-flow cases are less unstable at identical roughness Reynolds numbers. A variation of the wall cooling shows that the roughness-wake first- and second-mode behaviour is similar to that of the flat-plate flow: wall cooling stabilizes the first-mode and destabilizes the second-mode instabilities of the roughness wake.

Computational fluid dynamics simulations and wind tunnel measurements of unsteady wind loads on a scaled model of a very large optical telescope: A comparative study

Thorough theoretical and experimental investigations were performed to analyze the main characteristics of unsteady flows past a 1:100-scale wind tunnel (WT) model of a very large optical telescope housed within a spherical enclosure. The investigations were focused on the prediction and measurements of unsteady pressures on the inner and outer surfaces of the enclosure and on the telescope primary mirror. The WT measurements were performed essentially to provide aerodynamic data on the telescope structure and also to build a database for correlation with numerical simulation of the flow using computational fluid dynamics (CFD). Unsteady viscous flow solutions were computed for different telescope orientations using the lattice Boltzmann method coupled with the RNG k-ε turbulence model. For WT testing, unsteady pressure measurements were performed in an open jet WT for different telescope orientations and wind speeds, using a number of pressure taps distributed around the inner and the outer surfaces of the enclosure and on the primary mirror surface. A smoke stream visualization technique was also used to study the flow behavior around and inside of the telescope enclosure. The flow solutions were computed using the WT flow conditions. Correlations were obtained between CFD and WT data in terms of the mean pressure coefficients on the enclosure and the primary mirror surfaces, and for their standard deviations. Power spectral density analyses were also carried out for a number of pressure signals collected on the primary mirror surface. Both CFD solutions and WT measurements demonstrated that the flow inside and outside the enclosure was unsteady and massively separated on the back of the enclosure. The mean values and standard deviations of the pressure coefficients on the enclosure and the primary mirror surfaces correlated well with the experimental data. Using the WT Mach number in the simulation, the shear layer over the enclosure opening and the resulting acoustic wave effects were well captured, and there was excellent agreement between the CFD results and the WT measurements.

Einfluss der fahrbahnsimulation bei windkanal-versuchen an PKW

The influence of undersurface design on aerodynamic drag and aerodynamic lift is considered. Wind tunnel flow conditions and special test rigs like sting suspensions are studied, and rotating wheels and moving ground are simulated in model and full-scale tests. Pressure distributions in the wheel cavity and on the road surface are followed, and model measurements on the velocity profile close to the wheel dependent on undersurface shape are made. Comparison is made of the undersurface flow between wind tunnel tests and road tests for various undersurface configurations (venturi-shape of the front part and diffusor-like shape of the rear part). Analogy considerations are made concerning the flow profile close to the ground surface. Remarks are made on the wake flow behind cars.

Heat transfer in nonequilibrium dissociated hypersonic flow with surface catalysis and second-order effects.

An experimental investigation of heat transfer to hemispheres with both catalytic and noncatalytic surfaces in nonequilibrium dissociated hypersonic nitrogen flow has been conducted. The wind-tunnel flow conditions provided low Reynolds numbers, allowing the assumption of completely frozen shock and boundary layers while introducing the influences of second-order boundary-layer effects such as vorticity interaction. The results for copper surfaces, when correlated using the stagnation-point heat-transfer equation of Rosner together with Lees' heat-transfer distribution, are in good agreement with the second-order theory of Cheng under the assumption of complete surface recombination of atoms. Reductions in heat-transfer rate up to 40% were obtained with the use of noncatalytic coatings applied to the probes. Typically, values of the recombination coefficient range between 0.01 and 0.1 for the noncatalytic surfaces, compared with 1.0 for the catalytic surfaces.

Review of Flight and Wind-Tunnel Measurements of Boundary-Layer Pressure Fluctuations

A review is presented of available data on boundary-layer pressure fluctuations from flight and wind-tunnel investigations. The wind-tunnel data include flatplate pressure-fluctuation spectra and space-time correlations. The flight data include pressure-fluctuation spectra and “equivalent acoustic spectra,” the acoustic fields that would produce the same response as the aerodynamic fields. General trends are indicated by the results, and good agreement is found between flight and wind-tunnel measurements. Of particular interest is the fact that wind-tunnel measurements show the same scatter as flight measurements, in spite of the fact that wind-tunnel flow conditions are much better controlled and boundary-layer parameters are generally measured rather than estimated. For intercomparison purposes, the same normalization procedure has been used with all of the data. This has required the development of engineering curves by which boundary-layer parameters may be estimated. [Work sponsored by NASA-Langley.]