Pitfalls in the Interpretation of Delta Wing Flow Visualisation

This paper illustrates some pitfalls in the interpretation of flow visualisation images of delta wing leading-edge vortices. Numerical simulations of tracer particle trajectories in around straight and helical vortices are presented, representing unburst and burst delta wing vortices respectively. For straight vortices, the appearance of the ‘smoke-ring’ effect in wind tunnel testing is analysed, and shown to be unrelated to the size or presence of a viscous inner core in the vortex. The role of the swirl-induced radial pressure gradient in the vortex centreline trace seen in water tunnels is demonstrated. For helical (burst) vortices, the crossflow topology visualised with upstream tracer injection is shown to depend on the vortex core size and the helix pitch. Typical vortex helices are close to a flow regime which can produce visualised flow structures resembling axisymmetric bubble breakdowns. It is postulated that the intermittent open-ended bubble burst seen in delta wing vortices is in fact an artefact of the tracer flow visualisation technique, as transient disturbances temporarily reduce the pitch of the fundamental helical burst structure. List of Symbols Conical vortex a edge of outer vortex core, = r0/x dP diameter of tracer particle r, θ, x cylindrical coordinates re equilibrium (smoke ring) radius r0 radius of outer core uF, vF, wF axial, radial, tangential fluid velocities U0, V0 freestream axial and radial velocity α Hall vortex swirl parameter φ swirl ratio, = V0/U0 ρP tracer particle density ρF fluid density μ fluid viscosity τV viscous drag parameter τP density ratio, = ρP/ ρF Helical vortex a0 helix radius CMS binormal velocity coefficient CVT tangential velocity coefficient k0 helix pitch, = p/2π p helix pitch r, θ, z cylindrical coordinates R, Ψ, Z helical coordinates Paper presented at the RTO AVT Symposium on “Advanced Flow Management: Part A – Vortex Flows and High Angle of Attack for Military Vehicles”, held in Loen, Norway, 7-11 May 2001, and published in RTO-MP-069(I). Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 00 MAR 2003 2. REPORT TYPE N/A 3. DATES COVERED 4. TITLE AND SUBTITLE Pitfalls in the Interpretation of Delta Wing Flow Visualisation 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) NATO Research and Technology Organisation BP 25, 7 Rue Ancelle, F-92201 Neuilly-Sue-Seine Cedex, France 8. PERFORMING ORGANIZATION REPORT NUMBER 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release, distribution unlimited 13. SUPPLEMENTARY NOTES Also see: ADM001490, Presented at RTO Applied Vehicle Technology Panel (AVT) Symposium held in Leon, Norway on 7-11 May 2001 , The original document contains color images.