Eksperimen aliran visualisasi asap telah dijalankan untuk menangkap pembentukan vorteks dalam kaviti elips bagi sayap rasio aspek rendah (AR<4) pada α = 0º, 5º, 10º dan 15º. Eksperimen dijalankan pada kelajuan subsonik rendah 1.5m/s dengan sepadan bilangan Reynolds 26,000. Untuk mengkaji kesan bentuk pinggir depan kepada pembentukan vorteks, lima konfigurasi sayap dengan dua bentuk pinggir depan yang berbeza telah diuji (SCLE sayap model dan ELE sayap model). Bentuk kaviti untuk semua sayap model direka berdasarkan Yeung (2006) sebab vortex mempunyai trajektori elips. Vorteks ditarifkan sebagai vorteks stabil jika ia kekal mantap dalam kaviti dan ia mendorong lampiran aliran di pinggir belakang kaviti (Donelli et al., 2010). ELE sayap model telah menunjukkan potensi untuk memerangkap arah jam vorteks yang stabil dalam kaviti pertama pada α = 0º, 5º and 10º. Oleh itu, daya angkat dihasil oleh ELE sayap model dengan tiga kaviti adalah lebih tinggi berbanding ELE sayap model sebab daya angkat ditingkatkan oleh arah jam vorteks. Selain itu, SCLE sayap model telah memerangkap arah jam vorteks yang stabil dalam dua kaviti pertama pada α = 0º dan kaviti pertama pada α = 5º. Bagi SCLE sayap model dengan tiga kaviti, peningkatan daya angkat dalam dua kaviti pertama dijangka berlaku pada α = 0º. Tetapi, pada α = 5º, daya tekanan dihasilkan oleh vorteks tidak stabil dalam kaviti kedua dan ketiga telah mengantikan kesan peningkatan daya angkat oleh arah jam vorteks yang stabil dalam kaviti pertama. Dari segi keupayaan memerangkap vorteks, ELE sayap model dengan tiga kaviti mampu memerangkap arah jam vorteks pada jarak sudut serangan yang luas, dari 0º kepada 15º, manakala SCLE sayap model dapat memerangkap arah jam vorteks pada jarak sudut serangan yang lebih kecil, dari 0º kepada 10º. Disebabkan hidung jejari yang lebih besar, SCLE sayap model mengalami stall pada α = 15º. Oleh itu, keupayaan memerangkap vorteks boleh bertambah baik dengan mengurangkan hidung jejari. Keputusan juga menunjukkan bahawa kedua-dua SCLE dan ELE sayap model terdedah kepada vorteks tidak stabil yang gagal mendorong lampiran aliran dan mempunyai potensi untuk meningkatkan daya tekanan pada sudut serangan yang tinggi (α = 15º untuk SCLE sayap dan α = 10º untuk ELE sayap). Tetapi, peningkatan daya angkatan pada kedua-dua sayap model (SCLE dan ELE sayap) melalui vortex terperangkap boleh dilaksanakan pada sudut serangan tertentu.
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Smoke flow visualisation experiments were conducted to capture the vortex formation inside an elliptical cavity for a low aspect ratio wing (AR<4) at α = 0º, 5º, 10º and 15º. Experiments were conducted at low subsonic speed of 1.5m/s with corresponding Reynolds number of 26,000. In order to investigate the effect of leading edge shape to the vortex formation, five wing configurations with two different leading edge shape were tested (SCLE wing models and ELE wing models). The cavity shape for all wing models was designed based on Yeung (2006) as he revealed that vortex has an elliptical trajectory. Vortex is defined as a stable vortex if it stays steadily inside the cavity and it has sufficient energy to induce flow reattachment consistently at the trailing edge of cavity (Donelli et al, 2010). Based on current investigation, ELE wing model has shown the potential to trap a stable clockwise vortex in the first cavity at α = 0º, 5º and 10º. Hence, it is plausible that the lift generated by ELE wing model with triple cavities is higher than ELE baseline wing model due to the lift enhancement by clockwise vortex at α = 0º, 5º and 10º. Moreover, SCLE wing models have trapped stable clockwise vortex in first two cavities at α = 0° and first cavity at α = 5º. For SCLE wing model with triple cavities, the lift enhancement by clockwise vortex in the first two cavities is possible to occur at α = 0°. However, at α = 5º, the pressure drag generated due to unstable trapped vortices in the second and third cavities might have superseded the effect of lift enhancement by stable clockwise vortex in the first cavity. In addition, ELE wing model with triple cavities was found to be able to trap clockwise vortices for a wider range of angle of attack, varied from 0º to 15º whereas SCLE wing models are able to trap clockwise vortices for a smaller range of angle of attack, varied from 0º to 10º. Due to the larger nose radius design, SCLE wing models experienced stall at α = 15º. Hence, the investigation on vortex trapping capability can be widened by reducing the nose radius of leading edge. The results also revealed that both SCLE and ELE wing models are susceptible to unstable trapped vortices, which failed to induce flow reattachment and have the potential to increase the pressure drag, at higher angle of attack (α = 10º for SCLE wing and α = 15º for ELE wing). However, lift augmentation on both wing models (SCLE and ELE wing) by means of vortex trapping is feasible at certain angle of attacks.
Flow visualisation study of vortex formation for a wing with cavity / Tham Weng Hong