تدرج الرياح

تدرج الرياح في الاستخدام الشائع تدرج سرعة الرياح على وجه التحديد، أو بدلا من ذلك الرياح القص، هو المكون العمودي لتدرج متوسط سرعة الرياح الأفقية في الجو السفلي، ايضاً هو معدل زيادة قوة الرياح مع زيادة الوحدة في الارتفاع فوق مستوى سطح الأرض.^[1] في الوحدات المترية، غالبًا ما يتم قياسها بوحدات متر في الثانية من السرعة، لكل كيلومتر من الارتفاع (م / ث / كم)، مما يقلل إلى الوحدة القياسية لمعدل القص، الثواني العكسية (s⁻¹).^[2]

نبذة عدل

يجبر الاحتكاك السطحي للرياح السطحية على الإبطاء والدوران بالقرب من سطح الأرض، وتهب مباشرة نحو الضغط المنخفض، مقارنة بالرياح في التدفق شبه عديم الاحتكاك فوق سطح الأرض.^[3] تُعرف هذه الطبقة حيث يؤدي الاحتكاك السطحي إلى إبطاء الرياح وتغيير اتجاه الرياح، باسم طبقة حدود الكواكب.^[4] يؤدي التسخين الشمسي في النهار بسبب التشمس إلى زيادة سماكة الطبقة الحدودية حيث ترتفع درجة حرارة الرياح عن طريق ملامستها لسطح الأرض الساخن وتصبح مختلطة بشكل متزايد مع الرياح العالية.^[5] التبريد الإشعاعي بين عشية وضحاها يفصل تدريجيًا الرياح على السطح من الرياح فوق الطبقة الحدودية، مما يزيد من قص الرياح العمودي بالقرب من السطح، والمعروف أيضًا باسم التدرج الريحي.^[6]

الهندسة عدل

يجب أن يأخذ تصميم المباني حسبان أحمال الرياح، والتي تتأثر بتدرج الرياح. مستويات التدرج ذات الصلة التي يُفترض عادةً في رموز البناء، هي 500 متر للمدن و400 متر للضواحي أو المحافظات أو القرى، و300 متر للأراضي المفتوحة المنبسطة. للأغراض الهندسية يمكن تعريف ملف تعريف سرعة الرياح لقانون الطاقة على النحو التالي:^[7]

v_{z}=v_{g}\cdot \left({\frac {z}{z_{g}}}\right)^{1/\alpha },0<z<z_{g}

$v_{z}$ = سرعة الرياح في الارتفاع $z$
$v_{g}$ = رياح متدرجة عند ارتفاع التدرج $z_{g}$
$\alpha$ = معامل أسي

مراجع عدل

^ Hadlock, Charles (1998). Mathematical Modeling in the Environment. Washington: Mathematical Association of America. ISBN 978-0-88385-709-0. Thus we have a “wind-speed gradient” as we move vertically, and this has a tendency to encourage mixing between the air at one level and the air at those levels immediately above and below it.
^ Gorder, P.J.; Kaufman, K.; Greif, R. (1996). "Effect of wind gradient on the trajectory synthesis algorithms of the Center-TRACON Automation System (CTAS)". AIAA, Guidance, Navigation and Control Conference, San Diego, CA. American Institute of Aeronautics and Astronautics. ...the effect of a change in mean wind velocity with altitude, the wind velocity gradient...[permanent dead link]
^ Sachs, Gottfried (2005-01-10). "Minimum shear wind strength required for dynamic soaring of albatrosses". Ibis. 147 (1): 1–10. doi:10.1111/j.1474-919x.2004.00295.x. ...the shear wind gradient is rather weak....the energy gain...is due to a mechanism other than the wind gradient effect.
^ Oke, T. (1987). Boundary Layer Climates. London: Methuen. p. 54. ISBN 978-0-415-04319-9. Therefore the vertical gradient of mean wind speed (dū/dz) is greatest over smooth terrain, and least over rough surfaces.
^ Crocker, David (2000). Dictionary of Aeronautical English. New York: Routledge. pp. 104. ISBN 978-1-57958-201-2. wind gradient = rate of increase of wind strength with unit increase in height above ground level;
^ Jobson, Gary (1990). Championship Tactics: How Anyone Can Sail Faster, Smarter, and Win Races. New York: St. Martin's Press. pp. 323. ISBN 978-0-312-04278-3. You'll not recognize wind shear if your apparent wind angle is smaller on one tack than on the other because the apparent wind direction is a combination of boat speed and wind speed - and the sailing speed may be more determined by water conditions in one direction rather than another. This means that the faster a boat goes the more 'ahead' the apparent wind becomes. That is why the 'close reach' direction is the fastest direction of sailing – simply because as the boat speeds up the apparent wind direct goes further and further forward without stalling the sails and the apparent wind speed also increases – so increasing the boat's speed even further. This particular factor is exploited to the full in sand-yachting in which it is common for a sand yacht to exceed the wind speed as measured by a stationary observer. Wind shear is certainly felt because the wind speed at the masthead will be higher than at deck level. Thus gusts of wind can capsize a small sailing boat easily if the crew are not sufficiently wary.
^ Dalgliesh, W. A. and D. W. Boyd (1962-04-01). "CBD-28. Wind on Buildings". Canadian Building Digest. Archived from the original on 2007-11-12. Retrieved 2007-06-07. Flow near the surface encounters small obstacles that change the wind speed and introduce random vertical and horizontal velocity components at right angles to the main direction of flow.

روابط خارجية عدل

المكتبة الرقمية الوطنية للعلوم - رياح القص

تدرج الرياح في المشاريع الشقيقة:

[1] Hadlock, Charles (1998). Mathematical Modeling in the Environment. Washington: Mathematical Association of America. ISBN 978-0-88385-709-0. Thus we have a “wind-speed gradient” as we move vertically, and this has a tendency to encourage mixing between the air at one level and the air at those levels immediately above and below it.

[2] Gorder, P.J.; Kaufman, K.; Greif, R. (1996). "Effect of wind gradient on the trajectory synthesis algorithms of the Center-TRACON Automation System (CTAS)". AIAA, Guidance, Navigation and Control Conference, San Diego, CA. American Institute of Aeronautics and Astronautics. ...the effect of a change in mean wind velocity with altitude, the wind velocity gradient...[permanent dead link]

[3] Sachs, Gottfried (2005-01-10). "Minimum shear wind strength required for dynamic soaring of albatrosses". Ibis. 147 (1): 1–10. doi:10.1111/j.1474-919x.2004.00295.x. ...the shear wind gradient is rather weak....the energy gain...is due to a mechanism other than the wind gradient effect.

[4] Oke, T. (1987). Boundary Layer Climates. London: Methuen. p. 54. ISBN 978-0-415-04319-9. Therefore the vertical gradient of mean wind speed (dū/dz) is greatest over smooth terrain, and least over rough surfaces.

[5] Crocker, David (2000). Dictionary of Aeronautical English. New York: Routledge. pp. 104. ISBN 978-1-57958-201-2. wind gradient = rate of increase of wind strength with unit increase in height above ground level;

[6] Jobson, Gary (1990). Championship Tactics: How Anyone Can Sail Faster, Smarter, and Win Races. New York: St. Martin's Press. pp. 323. ISBN 978-0-312-04278-3. You'll not recognize wind shear if your apparent wind angle is smaller on one tack than on the other because the apparent wind direction is a combination of boat speed and wind speed - and the sailing speed may be more determined by water conditions in one direction rather than another. This means that the faster a boat goes the more 'ahead' the apparent wind becomes. That is why the 'close reach' direction is the fastest direction of sailing – simply because as the boat speeds up the apparent wind direct goes further and further forward without stalling the sails and the apparent wind speed also increases – so increasing the boat's speed even further. This particular factor is exploited to the full in sand-yachting in which it is common for a sand yacht to exceed the wind speed as measured by a stationary observer. Wind shear is certainly felt because the wind speed at the masthead will be higher than at deck level. Thus gusts of wind can capsize a small sailing boat easily if the crew are not sufficiently wary.

[7] Dalgliesh, W. A. and D. W. Boyd (1962-04-01). "CBD-28. Wind on Buildings". Canadian Building Digest. Archived from the original on 2007-11-12. Retrieved 2007-06-07. Flow near the surface encounters small obstacles that change the wind speed and introduce random vertical and horizontal velocity components at right angles to the main direction of flow.

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