Jenna L. Eppink

Innovative Flow Control Concepts for Drag Reduction

This paper highlights the technology development of two flow control concepts for aircraft drag reduction. The NASA Environmentally Responsible Aviation (ERA) project worked with Boeing to demonstrate these two concepts on a specially outfitted Boeing 757 ecoDemonstrator during the spring of 2015. The first flow control concept used Active Flow Control (AFC) to delay flow separation on a highly deflected rudder and increase the side force that it generates. This may enable a smaller vertical tail to provide the control authority needed in the event of an engine failure during takeoff and landing, while still operating in a conventional manner over the rest of the flight envelope. Thirty-one sweeping jet AFC actuators were installed and successfully flight-tested on the vertical tail of the 757 ecoDemonstrator. Pilot feedback, flow cone visualization, and analysis of the flight test data confirmed that the AFC is effective, as a smoother flight and enhanced rudder control authority were reported. The second flow control concept is the Insect Accretion Mitigation (IAM) innovation where surfaces were engineered to mitigate insect residue adhesion on a wings leading edge. This is necessary because something as small as an insect residue on the leading edge of a laminar flow wing design can cause turbulent wedges that interrupt laminar flow, resulting in an increase in drag and fuel use. Several non-stick coatings were developed by NASA and applied to panels that were mounted on the leading edge of the wing of the 757 ecoDemonstrator. The performance of the coated surfaces was measured and validated by the reduction in the number of bug adhesions relative to uncoated control panels flown simultaneously. Both flow control concepts (i.e., sweeping jet actuators and non-stick coatings) for drag reduction were the culmination of several years of development, from wind tunnel tests to flight tests, and produced valuable data for the advancement of modern aircraft designs. The ERA systems analysis studies performed by NASA indicated that AFC-enhanced vertical tail could produce approximately 0.9% drag reduction for a large twin aisle aircraft and IAM coatings could enable approximately 1.2% drag reduction recovery for a potential total drag reduction of approximately 3.3% for a single aisle aircraft with a natural laminar flow (NLF) wing design.

Flow Disturbance Measurements in the National Transonic Facility

Recent flow measurements have been acquired in the National Transonic Facility to assess the test-section unsteady flow environment. The primary purpose of the test is to determine the feasibility of the facility to conduct laminar-flow-control testing and boundary-layer transition-sensitive testing at flight-relevant operating conditions throughout the transonic Mach number range. The facility can operate in two modes, warm and cryogenic test conditions for testing full and semispan-scaled models. Data were acquired for Mach and unit Reynolds numbers ranging from 0.2≤M≤0.95 and 3.3×106<Re/m<220×106 collectively at air and cryogenic conditions. Measurements were made in the test section using a survey rake that was populated with 19 probes. Roll polar data at selected conditions were obtained to look at the uniformity of the flow disturbance field in the test section. Data acquired included mean total temperatures, mean and fluctuating static/total pressures, and mean and fluctuating hot-wire measurements...

Effects of Forward- and Backward-Facing Steps on the Crossflow Receptivity and Stability in Supersonic Boundary Layers

The effects of forward- and backward-facing steps on the receptivity and stability of three-dimensional supersonic boundary layers over a swept wing with a blunt leading edge are numerically investigated for a freestream Mach number of 3 and a sweep angle of 30 degrees. The flow fields are obtained by solving the full Navier-Stokes equations. The evolution of instability waves generated by surface roughness is simulated with and without the forward- and backward-facing steps. The separation bubble lengths are about 5-10 step heights for the forward-facing step and are about 10 for the backward-facing step. The linear stability calculations show very strong instability in the separated region with a large frequency domain. The simulation results show that the presence of backward-facing steps decreases the amplitude of the stationary crossflow vortices with longer spanwise wavelengths by about fifty percent and the presence of forward-facing steps does not modify the amplitudes noticeably across the steps. The waves with the shorter wavelengths grow substantially downstream of the step in agreement with the linear stability prediction.

Data Analysis for the NASA/Boeing Hybrid Laminar Flow Control Crossflow Experiment

The Hybrid Laminar Flow Control (HLFC) Crossflow Experiment, completed in 1995, generated a large database of boundary layer stability and transition data that was only partially analyzed before data analysis was abruptly ended in the late 1990s. Renewed interest in laminar flow technologies prompted additional data analysis, to integrate all data, including some post-test roughness and porosity measurements. The objective is to gain new insights into the effects of suction on boundary layer stability. A number of challenges were encountered during the data analysis, and their solutions are discussed in detail. They include the effect of the probe vibration, the effect of the time-varying surface temperature on traveling crossflow instabilities, and the effect of the stationary crossflow modes on the approximation of wall location. Despite the low turbulence intensity of the wind tunnel (0.01 to 0.02%), traveling crossflow disturbances were present in the data, in some cases at amplitudes up to 1% of the freestream velocity. However, the data suggest that transition was dominated by stationary crossflow. Traveling crossflow results and stationary crossflow data in the presence of suction are compared with linear parabolized stability equations results as a way of testing the quality of the results.

Interaction of a Backward-Facing Step and Crossflow Instabilities in Boundary-Layer Transition

A swept flat plate model with an imposed pressure gradient was experimentally investigated in a low-speed flow to determine the effect of a backward-facing step on transition in a stationary crossflow-dominated flow. Detailed hotwire measurements of boundary-layer flow were performed to investigate the upstream shift in transition due to a step height of 49% of the local unperturbed boundary-layer thickness. Increasing the initial stationary crossflow amplitude caused an upstream movement of the transition front for the backward-facing step case. The step caused a local increase in the growth of the stationary crossflow instabilities, but the stationary crossflow amplitude at transition was sufficiently low (<0.04Ue ) so that stationary crossflow was not solely responsible for transition. The unsteady velocity spectra downstream of the step were rich with unsteady disturbances in the 80- to 1500-Hz range. Three distinct families of disturbances were identified based on phase speed and wave angle, namely, a highly oblique disturbance (possibly traveling-crossflow-like), a Tollmien-Schlichting-wave-like disturbance, and a shear-layer instability. The stationary crossflow disturbances caused a modulation of the unsteady disturbances, resulting in spatially concentrated peaks in unsteady disturbance amplitude. This modulation of the unsteady disturbances is believed to be the reason for the upstream movement of the transition front with increasing stationary crossflow amplitude.

The Effect of Backward-Facing Step Height on Instability Growth and Breakdown in Swept Wing Boundary-Layer Transition

A low-speed experiment was performed on a swept at plate model with an imposed pressure gradient to determine the effect of a backward-facing step on transition in a stationary-cross flow dominated flow. Detailed hot-wire boundary-layer measurements were performed for three backward-facing step heights of approximately 36, 45, and 49% of the boundary-layer thickness at the step. These step heights correspond to a subcritical, nearly-critical, and critical case. Three leading-edge roughness configurations were tested to determine the effect of stationary-cross flow amplitude on transition. The step caused a local increase in amplitude of the stationary cross flow for the two larger step height cases, but farther downstream the amplitude decreased and remained below the baseline amplitude. The smallest step caused a slight local decrease in amplitude of the primary stationary cross flow mode, but the amplitude collapsed back to the baseline case far downstream of the step. The effect of the step on the amplitude of the primary cross flow mode increased with step height, however, the stationary cross flow amplitudes remained low and thus, stationary cross flow was not solely responsible for transition. Unsteady disturbances were present downstream of the step for all three step heights, and the amplitudes increased with increasing step height. The only exception is that the lower frequency (traveling crossflow-like) disturbance was not present in the lowest step height case. Positive and negative spikes in instantaneous velocity began to occur for the two larger step height cases and then grew in number and amplitude downstream of reattachment, eventually leading to transition. The number and amplitude of spikes varied depending on the step height and cross flow amplitude. Despite the low amplitude of the disturbances in the intermediate step height case, breakdown began to occur intermittently and the flow underwent a long transition region.

Flow Disturbance Characterization Measurements in the National Transonic Facility

Recent flow measurements have been acquired in the National Transonic Facility (NTF) to assess the unsteady flow environment in the test section. The primary purpose of the test is to determine the feasibility of the NTF to conduct laminar-flow-control testing and boundary-layer transition sensitive testing. The NTF can operate in two modes, warm (air) and cold/cryogenic (nitrogen) test conditions for testing full and semispan scaled models. The warm-air mode enables low to moderately high Reynolds numbers through the use of high tunnel pressure, and the nitrogen mode enables high Reynolds numbers up to flight conditions, depending on aircraft type and size, utilizing high tunnel pressure and cryogenic temperatures. NASA’s Environmentally Responsible Aviation (ERA) project is interested in demonstrating different laminar-flow technologies at flight-relevant operating conditions throughout the transonic Mach number range and the NTF is well suited for the initial ground-based demonstrations. In the current test, we acquired data for Mach and unit Reynolds numbers ranging from 0.2 ≤ M ≤ 0.95 and 3.3 × 10 6 < Re/m < 220 × 10 6 (1 × 10 6 < Re/ft < 67 × 10 6 )c ollectively at air and cryogenic conditions. Measurements were made in the test section using a survey rake that was populated with 19 probes. Roll polar data at selected test conditions were obtained to look at the uniformity of the flow disturbance field in the test section. Data acquired from the rake probes included mean total temperatures, mean and fluctuating static/total pressures, and mean and fluctuating hot-wire measurements. The results presented in this report focus primarily on the u

Observations of Traveling Crossflow Resonant Triad Interactions on a Swept Wing

†Experimental evidence indicates the presence of a triad resonance interaction between traveling crossflow modes in a swept wing flow. Results indicate that this interaction occurs when the stationary and traveling crossflow modes have similar and relatively low amplitudes (~1% to 6% of the total freestream velocity). The resonant interaction occurs at instability amplitudes well below those typically known to cause transition, yet transition is observed to occur just downstream of the resonance. In each case, two primary linearly unstable traveling crossflow modes are nonlinearly coupled to a higher frequency linearly stable mode at the sum of their frequencies. The higher-frequency mode is linearly stable and presumed to exist as a consequence of the interaction of the two primary modes. Autoand cross-bicoherence are used to determine the extent of phase-matching between the modes, and wavenumber matching confirms the triad resonant nature of the interaction. The bicoherence results indicate a spectral broadening mechanism and the potential path to early transition. The implications for laminar flow control in swept wing flows are significant. Even if stationary crossflow modes remain subcritical, traveling crossflow interactions can lead to early transition.

The Effect of Forward-Facing Steps on Stationary Crossflow Instability Growth and Breakdown

The effect of a forward-facing step on stationary crossflow transition was studied using standard stereo particle image velocimetry (PIV) and time-resolved PIV. Step heights ranging from 53 to 71% of the boundary-layer thickness were studied in detail. The steps above a critical step height of approximately 60% of the boundary-layer thickness had a significant impact on the stationary crossflow growth downstream of the step. For the critical cases, the stationary crossflow amplitude grew suddenly downstream of the step, decayed for a short region, then grew again. The adverse pressure gradient upstream of the step resulted in a region of crossflow reversal. A secondary set of vortices, rotating in the opposite direction to the primary vortices, developed underneath the uplifted primary vortices. The wall-normal velocity disturbance (V ′) created by these secondary vortices impacted the step, and is believed to feed into the strong vortex that developed downstream of the step. A large but very short negative crossflow region formed for a short region downstream of the step due to a sharp inboard curvature of the streamlines near the wall. For the larger step height cases, a crossflow-reversal region formed just downstream of the strong negative crossflow region. This crossflow reversal region is believed to play an important role in the growth of the stationary crossflow vortices downstream of the step, and may be a good indication of the critical forward-facing step height.

The Effect of Acoustic Forcing on Instabilities and Breakdown in Swept-Wing Flow over a Backward-Facing Step

Instability interaction and breakdown were experimentally investigated in the flow over a swept backward-facing step. Acoustic forcing was used to excite the Tollmien-Schlichting (TS) instability and to acquire phase-locked results. The phase-averaged results illustrate the complex nature of the interaction between the TS and stationary crossflow instabilities. The weak stationary crossflow disturbance causes a distortion of the TS wavefront. The breakdown process is characterized by large positive and negative spikes in velocity. The positive spikes occur near the same time and location as the positive part of the TS wave. Higher-order spectral analysis was used to further investigate the nonlinear interactions between the TS instability and the traveling crossflow disturbances. The results reveal that a likely cause for the generation of the spikes corresponds to nonlinear interactions between the TS, traveling crossflow, and stationary crossflow disturbances. The spikes begin at low amplitudes of the unsteady and steady disturbances (2-4% Ue) but can achieve very large amplitudes (20-30% Ue) that initiate an early, though highly intermittent, breakdown to turbulence.